Instrumentation amplifier

Transcription

1 Instrumentationamplifieris a closed-loop gainblock that has a differential input and an output that is single-ended with respect to a reference terminal. Application: are intended to be used whenever acquisition of a useful signal is difficult. The precision of an IA is provided at the expense of flexibility. Performance of an IA are optimized performance for data acquisition and it is not useful for mathematical operations like op-amp. Requirements:IA is designed to amplify very low sensor signal (of the order of µ) and simultaneously to reject high common-mode signal (of the order of ).

2 properties: Differential input and single-ended output Gain can be adjusted by an internal or external gain resistor ery high input impedance. Input impedances of an in-amp are closely matched to one another. The typical value of the input impedance is about 10 9 Ω. ery low output impedance, of the order of mω. ery low bias currents. typical values of input bias current for bipolar inamp range from 1nA to50na. ery high CMRR, whose typical values are between 70 do 100dB.

3 Similarities between the operational amplifier and the instrument amplifier Differential input Single-ended output High input impedance (abour10 9 Ω) Low output impedance Negligible input bias currents (typically of the order of na) Differences between instrumentation amplifier and operational amplifier: In-amp has an internal feedback network. In-amp gain is adjustable by a gain resistor In-amp gain is not very large like in op-amp In-amp is exclusively intended for the amplification of the differential signal.

4 Advantages of the instrumentation amplifier An instrumentation amplifier is a closed-loop gain circuit whose feedback is implemented inside the integrated circuit and isolated from its signal input terminals. As opposed to the inamp, op-am linear circuits use external resistors connected between op-amp output and inverting input. Advantages of the instrumentation amplifiers: In-amp cancel out AC as well as DC signals that are common to both inputs as opposed to the op-amp circuits which usually simply amplify both the signal voltage and any dc, noise, or other common-mode voltages. In-amp gain can be adjusted in a wide range by an internal or external gain resistor which is isolated from the signal inputs. This resistance as opposed to the gain resistors in op-amp circuits, does not affect CMRR..

5 Data acquisition The most important application of the instrumentation amplifier is extracting small signals from transducers and other signal sources. In-amps find their primary use amplifying signals from low level output transducers in noisy environments. The measurements of temperature, pressure, weight usually include bridge circuit.

6 Typical applications for an instrumentation amplifier Medical Instrumentation In-amps are widely used in medical equipment such as EKG and EEG monitors, blood pressure monitors. Software-Programmable Applications An in-amp may be used with a software-programmable resistor chip to allow software control of hardware systems. Audio Applications Because of their high common-mode rejection in- amp are used in audio applications to extract a weak signal from a noisy environment. ideo Applications High speed in-amps may be used in many video and cable RF systems to amplify or process high frequency signals. Monitor and Control Electronics Diff amps may be used to monitor voltage or current in a system and then trigger alarm systems.

7 Properties of an instrumentation amplifier Common-Mode Rejection This is the most important characteristic of an instrumentation amplifier. CMR should be high over the range of input frequencies that need to be rejected. Offset oltage and offset voltage drift. total output offset will equal the sum of the gain times the input offset plus the offset of the output amplifier. Although the initial offset voltage may be nulledwith external trimming, offset voltage drift cannot be adjusted out. Input ImpedanceThe impedances of the inverting and noninvertinginput terminals of an in-amp must be high and closely matched to one another. High input impedance is necessary to avoid loading down the input signal. Typical values of input impedance is between 10 9 Ωto 10 1 Ω. Input Bias and Offset Current Errors are inevitable. Typical values of input bias current for bipolar in-amp range from 1nAto 50nA. Note that if the input source resistance becomes infinite, as with ac (capacitive) input coupling, the input common-mode voltage will climb until the amplifier saturates. This problem is prevented with a high value resistor connected between each input and ground.

8 Properties of an instrumentation amplifier Noise.Becauseitmustbeabletohandleverylowlevelinputvoltages,an in-ampmustnotadditsownnoisetothatofthesignal. Nonlinearity is an inherent performance limitation of the device and cannot be removed by external adjustment. Nonlinearity is normally specified as percentage of full scale. Gainselectionshouldbeeasy.Theuseofasingleexternalgainresistor is common, but an external resistor will affect the circuit s accuracy and gain drift with temperature. The better solution is the implementation of two resistors for gain selection where gain is proportional to the ratio of these two resistance. Bandwidth An instrumentation amplifier must provide bandwidth sufficient for the particular application. Typical unity gain, fall between 500 khz and 4 MHz. Small-signal bandwidths, performance at low gains is easily achieved, but at higher gains bandwidth becomes much more of an issue.

9 Standard instrumentation amplifier contains three op-amp.

10 The first stage of the instrumentation amplifier is a non-inverting follower configuration. This configuration offer an extermelylarge input impedance. I D 1 IN+ IN IN+ R G I + I D IN D R F R DG D 1 CM 1 CM + G 1 Where G1 is the gain of the first stage G 1 R 1+ R We can notice that first stage amplifies only differential signal while common mode signal transfer unchanged at the output. F G F R D G

11 Extending the common mode voltage range When using instrumentation amplifier, it is necessary to take care about the value of the common-mode component of the input signal. Namely, at a certain value of CM the input op-amps reached the saturation. Both input operational amplifiers, A1 and A are in non-saturated mode when the following condition is satisfied: 1, CM + G1 D The range of the common mode voltage can be increased by decreasing thegainoftheinputstageg1: 1, ' const. D G1 D CM + CM + G sat ' 1 ' CM > CM ' 1 G1 G <

12 Extending the common mode voltage range IfordertoreducethegainofthefirststageG1andpreservethetotalgain Gtot,itisnecessarytoincreasethegainofthesecondstageG. When we reduce input voltage gain from G1 to a smaller gain of G1 the gain of the second stage should be changed from unity gain to: G ' G TOT ' G 1 ' ' R G R 1 Two feedback resistances of the differential amplifier R are usually realized as a serial connection of an internal resistance R and an external resistance Rext. This solution is not suitable for larger gain because in that case external resistors Rext, significantly increase the level of noise.

13 Extending the common mode voltage range Thecommonmodevoltagerangecanbeexpandedbyusingabuffered voltage divider in a feedback path form the output of A3 to its inverting input. In this case the output voltage is determined as: R R R1 R 3 This solution provides a wide range of gain with moderate resistor values ofr3andr4.furthermore,itpreservesahighvalueofcmrrasopposed to the previous solution with external resistors. 4

14 Rejection of the differential DC signal The signal conditioning in some applications is a challenge task due to the presence of a large differential DC signal and very small AC signal. In medical equipment such as EKG and EEG monitors, signal is sensed by electrodes on the skin surface. These electrodes typically generate a common-mode voltage of 1.5. The differential component of the signal is composed of DC voltage of about ±500 m and AC voltage between 0.5m and 1.5 m. The use of an active filter in front of an instrumentation amplifier is not a good solution because it decreases CMRR. The detection of small AC signal in the presence of large differential DC potential is performed in three steps: 1. Reduction of the input stage gain G1 in order to avoid saturation of the input operational amplifiers A1 i A.. Apply low-pass filtering in the output stage to remove the differential DC signal. 3. Apply high gain in the output stage to increase the level of the AC signal.

15 Rejection of the differential DC signal One way to reject DC differential signal is to add a buffered voltage divider and a low pas filter in the output stage of the instrumentation amplifier. The voltage divider feedbacks signal from the output to the inverting input anditspurposeistoenlargethegainoftheoutputstage. Thelowpassfilterreturnsignalfromtheoutputtothenon-invertinginput anditisusedtorejectdcdifferentialsignal.

16 oltagesattheop-ampinputscanbeexpressedas: After equating potentials at the inputs of the op-amp A3, we get the expression for the voltage gain: Rejection of the differential DC signal R R R n int int 0 1 C R j p ω ' 0 f 0 f j Where: + ' f f j f G A n 3 1 R R 4 G + int int ' 0 1 G C R f π

17 Rejection of the differential DC signal The alternative solution to reject differential DC signal is take the second stage gain of G1 and to introduce a separate gain stage. This gain stage contains an integrator and an inverting amplifier. The integrator composed of A5, Rint and Cint performs low-pass filtering. The final gain stage composed of op-amp A4 introduces additional gain and performs low-pass filtering of high-frequency noise.

18 Day 4 Problem 1 Design an electrocardiograph (ECG) amplifier by using an instrumentation amplifier with three rail-to-rail op-amps whose voltagesupplyis±5.theamplifiershouldbeabletoamplifythe following signal: common-mode voltage of 1.5, DC differential voltage of 0.4, AC differential voltage of 1m. Lower 3-dB frequency should be 0.05 Hz.