Select the Right Operational Amplifier for your Filtering Circuits

Transcription

1 Select the Right Operational Amplifier for your Filtering Circuits 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 1 Hello, my name is Bonnie Baker, and I am with Microchip. Today, we are going to talk about selecting the right amplifier for your filtering circuit. As a matter of fact, selecting the right amplifier for your low-pass filtering circuit. for Low Pass Filters Page 1

2 Analog Filters Data Acquisition System Digital Vs Analog Filters - Pros and Cons Nyquist Theorem Low Pass Filter vs Frequency Analog Low Pass Filter Circuits Amplifier Specifications The Right Amplifier for Your Filter 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 2 The topics of today are shown on this slide, and there seven of them listed there, but starting out with the first topic of data acquisition system. I am going to give you a block diagram of where the low-pass analog filters fit in the system. Ninety-nine percent of the time, a low-pass filter will be found in this kind of system. So, this is an appropriate topic to talk about; but you can design them into other systems, so keep that in mind. Following the discussion of the data acquisition system, we will talk about digital and analog filters. And I don t know about some of you, but I prefer to design digital filters if I can, but it turns out that analog filters are really quite critical in your system if you want to have good fidelity. This will spill into the topic of Nyquist Theorem, and will talk about why you need that analog filter, as opposed to the digital one; and then following that with the frequency response of an analog low-pass filter. Now, an analog, as you know, that we will talk about frequency. Additionally, you might be used to talking about the time zone about your signals, but in analog will be talking a lot about frequency responses. We will follow this frequency discussion with actual circuits that you can use to build low-pass filters, and you can go into your lab today, and can build the circuits that I am mentioning and talking about. So the punch line and the intent of this presentation is the amplifier specifications, and how do you choose the right amplifier for your filter. I am going to give you a few tips on how to do that. You may look at the amplifier data sheets and notice there are 20, 30, 40 facts in the table fact sheets, as well as graphs. And the question is which one of these specs is important when you design an analog filter. for Low Pass Filters Page 2

3 Data Acquisition System Analog Gain and Signal Conditioning Cell Analog Low Pass Filter Analog to Digital Conversion Microcontroller w/digital Filter Input Signal Analog Output Signal Digital 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 3 So, let s talk about the data acquisition system. Starting out at the far left of the slide, a signal comes into the analog gain and signal conditioning cell, and gain a curve or it might be a differential input from an instrumentation amplifier or something like that. But a signal does come it, and after you condition it, I highly recommend that you put in an analog lowpass filter. And, that is the second block, here. That low-pass filter then sends a signal into the A to D converter. The A to D converter can be external to the micro controller or internal, it doesn t matter. And it doesn t matter what technology you use to do that conversion, whether it s the SAF or Successive Approximation File, or some kind of sampling system. Regardless, of that system, that data then goes into the micro-controller, and at that point, you can take that data and massage it and multiply it, or add it, or average it, or whatever inside the micro-controller with the digital filter. So, the question that I would ask is: If I have a digital filter, why would I need an analog low-pass filter? Maybe I could solve just all of my problems in the digital domain. A lot of people tell me that is what they would try to do. for Low Pass Filters Page 3

4 Filtering Pros and Cons Analog Filter - Hardware Implementation Pros Eliminates Aliasing Problems Improved Signal to Noise Ratio: Out-of-band noise Cons Noise Floor Limited by Electronics Digital Filter - Firmware Implementation Pros Increases Signal to Noise Ratio: In-band noise Easier to Program Frequency Response Cons Slow Response Time, Latency 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 4 Let us talk about the analog filter, first of all. Analog filters are hardware implementation, which makes them kind of awkward, if you want to make changes in your filter type. You have to pull out capacitors or resistors and change the performance of that filter in that manner. But, the good side of this whole thing is it eliminates aliasing problems. I am going to talk about that in just a second, and what that means. And an improved signal to noise ratio in the out-of-band area. Wherever you have your sampling frequency for your A to D converter this improves the signal to noise ratio at higher frequencies than that sampling frequency. On the down side, the noise floor is limited by your analog electronic. And I also mention the hardware portion of the implementation that is kind of awkward. Let us talk about digital filters, because while everybody is saying this is the best way to go, it is a far more implementation, and the truth of the matter is, you can change the filter response on the fly very quickly with a few key strokes at your computer and reprogram your processor or controller and you are good to go. It also increases the signal noise ratio on the in-band noise, and that means that the signal noise ratio in the region from dc up to your sampling frequency and beyond reduces that noise down. On the down side of this, however, the digital filters is the response time is slow. There is latency. You have to take a lot of code in; a lot of conversions in, and run a lot of calculations if you are averaging or doing an IRR filter, or whatever. That takes time and the latency could become an issue in your circuit. Let s go back to the analog filter. Why is that analog filter needed? That issue will lead into the Nyquist Theorem. for Low Pass Filters Page 4

5 Nyquist Theorem Harmonics that Fold Back into the Conversion Output Analog Input N = 0 (1) (2) N = 1 N = 2 (3) N = 3 f 0 S /2 3f f S /2 5f S 2f S /2 S 3f S Sampled Output Representation N = 0 (1) (2) (3) 0 f S /2 f S f ALIASED = f IN - Nf S Find N by making f ALIASED < f s / Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 5 If you look at the slide there are 2 graphs on the side. The one on the left-hand side is a representation of the analog signal and the frequency domain. The graph on the right-hand side is a representation of digital out-put code with frequency in terms of frequency; but, let me explain this a little bit more in detail. Let s look at the left-hand graph. The X-axis is frequency, zero to F S over 2, 3 F S over 2, etc., etc. F S is a variable that tells you that t hat is the sampling frequency of your A/D converter. That is a really important thing to understand and know. On the Y axis is the analog input and this is analog, it is not digital; it is purely analog. So, this is the signal that is going into your A/D converter. Now, inside this graph is a lot of arrows and numbers 1, 2 and 3. These arrows represent a signal that has that frequency content. For instance, arrow numbered 1, we will talk about in a second. That signal comes in at a frequency lower than half of the sampling frequency, and the magnitude you can see is up about half way up the scale. Signal #2 is an interesting one. It is a signal that happens to have the lower half of the sampling frequency, and the magnitude is not as high as signal #1, but regardless, that is what it looks like. And signal #3 is well above two times the sampling frequency. Now if you run an A/D converter, you might want to assume that if your sampling frequency is 10,000 samples per second, you would think that all of the frequencies above 10,000 samples per second would not be converted and that is not true. That is absolutely not true. What actually happened and I am looking at the graph at the right-hand side; particularly, #1 was below half of the sampling frequency come through reliably. The right magnitude and the right frequency., but signal #2 comes through with a contaminated frequency. The component was contaminated. You will notice that it was above half of the sampling frequency, now it is below. And the magnitude is preserved. Look at signal #3. The same problem. The frequency component is contaminated; the magnitude is preserved. And if look at that hard in the digital domain and decide which is the good, which is the bad and which is the ugly. You can t differentiate that out. How do we deal with this thing?? In the analog domain, we put in a filter for Low Pass Filters Page 5

6 0dB Buffered Amplifier Frequency Response Bandwidth of buffered MCP601 20dB 40dB 60dB 80dB 10Hz 1kHz 100kHz 10MHz 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 6 I am going to talk about the analog filter, starting out with a simple amplifier. This amplifier is represented in the frequency domain and the X axis again is the frequency and the Y axis is dv or magnitude. The amplifier that I am showing here is the MCP601. The bandwidth of this amplifier is 2.8 MHz and you can see where the corner frequency of that band width that I we re pointing to. This particular amplifier I configured in a buffer and a gain of 1 volt per volt. So at DC, the gain is zero db; and as you go up in frequency, it continues to be zero to d, until you come to around 2.8 MHz, and the gain starts to go down because of the amplifier. That is nice. The amplifier gives you a single volt filter, and gets rid of all of the noise above, somewhere around 2.8 MHz and higher; however, there is a lot of noise there. What are we going to do with the rest of the noise? for Low Pass Filters Page 6

7 0dB Low Pass Analog Filter Response Bandwidth of buffered MCP601 20dB 40dB 2nd Order, 10Hz Filter Noise Eliminated by 2nd Order Filter 60dB 80dB 10Hz 1kHz 100kHz 10MHz 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 7 We re going to throw in a low-pass filter. In the slide you will see that I m throwing in the design of the second order, a 10 Hz filter. I am using the same amplifier but I am putting capacitors, and resistors around that amplifier to design the second order 10Hz filter. The noise that is eliminated by the second order filter, shown in the gray area of this slide. This does not infer your sampling frequency had to be brought down to 10Hz. Not at all. Your sampling frequency can be at a KHz, or 10Khz or whatever. But, you have gotten rid of that noise. And that noise will cause you some problems in looking at your digital code on the output of the A/D converter. This is a nice solution in getting rid of that noise. for Low Pass Filters Page 7

8 Typical Analog Low Pass Filters Sallen Key At DC the gain is Positive If DC gain is +1V/V the Input Common mode voltage of amplifier should to be Rail-torail V IN + V OUT Multiple Feedback At DC the gain is Negative Requires a Voltage Reference in Single Supply Applications V IN V REF FilterLab Active Filter Design Software + V OUT 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 8 Now let me give you some neat circuit designs you can go into the lab and try out. These are common circuits we use in technology to design low pass filters. The first one is the Sallen Key filter. The second is the Multiple feedback circuit. I will go into a little more detail in a second. But, understand, that you can design low-pass filters in a lot of ways. These are the most common ways of doing it. Like I said, you can design it with RC, and a series of RC s and implement it with the same kind of function. Let s talk about the Sallen Key, a very common filter. At DC the gain is positive and let me see if I can walk you through this. If I start on this circuit, starting with Vin, and follow that line on through to the amplifier, you ll see that I m going into the non-inverting input of the amplifier. Now, at DC, as we all know, capacitors act like open circuits. So imagine that the two capacitors in the circuit don t exist; they re gone; they re not there. And, so you look at that circuit now and you see a buffer and the gain is one volt per volt gain on this system. So, if the DC gain is one volt per volt, you have to think about it for a second; that input is going to be running all over the place, depending on your input voltage. So, it is important to have common mode voltage range on the input of the amplifier to be rail-to-rail. And we are going to revisit that spec. It is an important spec for this kind of service. The multiple feed back on the second type of circuit and in this kind of situation, the DC gain of the low-pass filter is negative. If you follow thru on the Vin input once again, and follow thru at the amplifier, you will notice it will go into the inverting input, and if you remove the capacitor from that circuit, you will immediately say, yeah that s an amplifier in a negative gain. The downside of the circuit is it requires a voltage reference in single supply application. Now, if you are doing a dual supply application that reference should be ground. But in single supply application you have to have a voltage reference; frankly, I put a resistor from the supply to the reference pin and then down to ground and I have a voltage divider, and boom, I have my reference. It is easy to do. There are a lot of resistors to both of these circuits and I argue with myself should I give you the full-blown formula on figuring out the corner frequencies with these resistors and capacitors, so you can calculate yourself, and the answer was, no. I am going to recommend you use a software program, called FilterLab. It is an active filter design software and you go in there, and you can download it from Microchip s website. You go in, and you use the software, and you will be able to design filters in a snap. In a blink of an eye; very, very quickly. And, oh, by the way, it is free. for Low Pass Filters Page 8

9 Specifications of Merit INPUT Input Bias Current, I B Input Impedance, Z IN Common Mode Input Voltage Range, V CM Input Noise, e n Offset Voltage, V OS Common-Mode Rejection Ratio, CMRR V DD POWER SUPPLY Power Supply Voltage, V DD Power Supply Current, I DD Power Supply Rejection Ratio, PSRR V IN- V OUT SIGNAL TRANSFER DC Open Loop Gain, A OL Gain Bandwidth Product, GBWP Phase Margin Slew Rate, SR Settling Time Harmonic Distortion V IN+ V SS OUTPUT Output Swing, V OUT Output Short Circuit Current, I SC Output Impedance, Z OUT 18 specs 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 9 Now, you have the filters. You have a tool to help you design that filter. But, the real reason we re having the seminar today is How do you select the right amplifier for your filter circuit? What I have done on the side is list a bunch of specifications, and the input, the power supply, the output, the signal transfer. And I m pointing to the bottom, and this is 18 specs. And this particular amp part is not specified really well, and I find data sheets that have 20, 30, 40 specs in their tables. How do you dig through this stuff and figure out what you need to know and not. And the answer to that question is, if you look at the sides, there are 2 specs that are high-lighted in yellow. The gain bandwidth product and slew rate. The answer to your question is how to select an amplifier. You select the amplifier with those two specs in line with the filter you re designing and you will have a solution. It will work. The important specs again are the gain bandwidth product, slew rate. The secondary specs that I will talk about and touch on. for Low Pass Filters Page 9

10 Important Op Amp Specifications for LP Filters Primary Specifications Gain Bandwidth Product For Sallen Key: GBWP > 100*G CLN * f C, For Multiple Feedback: GBWP >100*(-G CLI + 1)*f C, Slew Rate Slew Rate > (2π V OUT P-P * f C ), Secondary Specifications Input Common-Mode Range Rail to Rail Input for Sallen Key Filters Input Bias Current Pico Amp input bias current required if f C < 1000Hz 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 10 Let s talk about the important ones, the primary specifications. First of all, gain bandwidth product. For Sallen key filter. If you select an amplifier that has a gain bandwidth product that is greater than 100 times the gain of a close loop system in your amplifier, multiply that by the corner frequency of your filter; those three elements, and your gain bandwidth product is larger than the formula then you have an amplifier that will not interfere with the performance of your filter and everything will work well. That is a good success story. For the multiple feedback amplifier, you select the gain bandwidth product with the amplifier could be greater than 100 times the closed loop gain of the inverted amplifier plus one and that quantity times the corner frequency, and once again, you will have a successful circuit filter. The second specification that you have to meet is the slew rate. Slew rate has to be greater than 2pi, the output voltage, peak-to-peak, times the corner frequency, once again. Now, if you meet these 2 sets of requirements and you select an amplifier, you are 90% there in providing a very successful low-pass filter amplifier circuit. Secondary specifications. The input common-mode range; the Sallen key filters is important that it be rain-to-rail input, as we discussed earlier, and another secondary specification is the input bias current. It has to be in the pico input bias current, has to be in the pico amp range. The desired required cut-off frequency is less than a KHz. The reason for that is that you have higher resistors in your filtering circuit and that bias current will flow through the resistors and create an offset voltage. for Low Pass Filters Page 10

11 Op Amps Appropriate for Low Pass Filter Designs 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 11 Ok, so, here we go, I have to throw this slide up here and you know that filters can be built with a lot of amplifiers and made by a lot of manufacturing. Luckily, Microchip has a dozen or so amplifiers that will fit the bill. I can say something about the MCP6041. You notice the gain bandwidth product that 6041 is 14kHz, and from the formula that we went over prior, I would not design a filter that had a bandwidth of 10kHz. That wouldn t work. I would have to design a filter with a lower frequency, and you can quickly see how that worked. The columns I have in this graph are the gain bandwidth product, slew rate, input common mode voltage and input bias current at room temperature. That is a good reference. Now, how do you get a little bit further. This is a 20-minute seminar, and it doesn t give you a lot of in-depth information, but let me tell you where to look. The ADN003 is a app note that you can find on the website after this or you might have it already and that is what this seminar was based on; the ADN003. The AN699 is a very in-depth discussion about anti-aliasing filters for data acquisition systems, and that would be a good place for you to start in reading. In terms of feature articles: EDN, February 20, 2003, Filtering, before or after. That talks about the difference between digital and analog filtering, when to use them and when not to use them. Nice article. And, finally, probably the best reference you can do is download: Filter Lab Active Filter Design Software. It is down-loadable, like I said, there at our website and this will help you design active filters. for Low Pass Filters Page 11

12 Application Notes References ADN003, Select the Right Operational Amplifier for your Filtering Circuits AN699, Anti-Aliasing, Analog Filters for Data Acquisition Systems AN723, Operational Amplifier AC Specifications and Applications AN695, Interfacing Pressure Sensors to Microchip s Analog Peripherals AN737, Using Digital Potentiometers to Design Low Pass Adjustable Filters Feature Articles EDN, February 20, 2003, Filtering? Before or after? FilterLab Active Filter Design Software Down-loadable at Active, op amp filters 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 12 Now, how do you get a little bit further? This is a 20-minute seminar, and it doesn t give you a lot of in-depth information, but let me tell you where to look. The ADN003 is an app note that you can find on the website after this or you might have it already and that is what this seminar was based on; the ADN003. The AN699 is a very in-depth discussion about anti-aliasing filters for data acquisition systems, and that would be a good place for you to start in reading. In terms of feature articles: EDN, February 20, 2003, Filtering, before or after. That talks about the difference between digital and analog filtering, when to use them and when not to use them. Nice article. And, finally, probably the best reference you can do is download the Filter Lab Active Filter Design Software. It is down-loadable, like I said, there at our website ( and this will help you design active filters. Now, I haven t gone into details about this so I ll do it quickly. It will help you design your low-pass filters, as we discussed. It will help you design high-pass filters which is a nice feature, and finally it will help you get through band-pass filter design. Very nice to use for all of your filtering needs. for Low Pass Filters Page 12

13 Conclusion Data Acquisition System Nyquist Theorem Analog Low Pass Filter Circuits Amplifier Specifications Choose the Right Amplifier for Your Filter Design your Circuit using FilterLab Active Filter Design Software 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 13 Alright, let s summarize what we have talked about. We talked about the data acquisition system and why low-pass filters are required, and that is filtering through the Nyquist Theorem. And how signals if you don t use a low-pass analog filter how alias signals will fall back into your digital data.. We looked at a couple of circuits, low-pass circuits, the Sallen Key in the multiple feedback and finally we took a look at the amplifier specifications. That is why we came here today to discuss the topics. Finally, we figured out a way to choose the right amplifier for your filter, and we recommend at the final note that you download Filter Lab, the active filter design software. That concludes this presentation. for Low Pass Filters Page 13

14 Select the Right Operational Amplifier for your Filtering Circuits 2003 Microchip Technology Incorporated. All Rights Reserved. for Low Pass Filters 14 for Low Pass Filters Page 14