Differential Amplifiers
Benefits of Differential Signal Processing The Benefits Become Apparent when Trying to get the Most Speed and/or Resolution out of a Design Avoid Grounding/Return Noise Problems Better Distortion/Dynamic Range For the same Amplitude Differential Signal the Outputs do not Swing as Close to the Rail Lower Distortion especially the 2nds Analog Signals in High-performance Systems Start and End Differential Almost Always the Signal Source from the Real World is Differential High-speed ADCs Have Differential Inputs 2
Single-ended Components Cannot Reject Ground Noise Each Part of the Circuit Has a Different Reference Point No Matter How Careful you are with Grounding High Frequency Ground Currents will Cause Some Problems which May be Difficult to Work Around Op Amp Can not Reject This Ground Noise V SIGNAL Z GND I NOISE GND 1 GND 2 3
Differential Amps Have Effective CMRR Differential Signal does not Need a Reference Ground and Other Noise Sources are Common to Both Inputs CMRR of Differential Amp is Effective V SIGNAL V OCM V OCM GND 1 Z GND I NOISE GND 2 4
Why Differential Signal Processing is not More Common Differential Signals are Commonly used Today for ADC and Line Driving Differential Signaling is not Generally Considered for Other Uses Because: Discrete Differential Designs can Be Difficult to Implement Some Applications can not Tolerate the Higher Cost Not Many Differential ICs are Available Transformers must be Used As Speeds and Resolution Increase the Benefits of Differential Signaling Become More Necessary 5
Differential Input/Output High-speed Amps AD8131/2/8
High Speed Differential Amps for Challenging Designs V S+ V OCM V OUT+ V OCM V OUT, Single ended V OUT- V OUT,Differential V S- Differential Signal Processing Simplifies Circuit Design Avoid Ground Noise 2x Dynamic Range of Op Amps Balanced Outputs Minimize EMI High CMRR Reduces EMI Susceptibility High Speed ADC Driving ADCs Perform Better when Driven Differentially Like a Voltage Feedback Op Amp: Gain Set by Ratio of R F /R G Signal Gain, Filtering, Level Shifting, Buffering / Driving 7
What s Inside the AD8131/2/8 Diff Amps? Internal CM Feedback forces Forces both outputs to be balanced, Equal in amplitude 180 out of phase: V OUT, CM = (V OUT+ + V OUT- )/2) Balance is unaffected by R F /R G matching Differential feedback effectively creates 2 summing nodes Forces Both Inputs to the same voltage when the loop is closed High Input Z, Low Output Z R F 3 5 V OUT- V IN+ V IN- R G 8 1 2 V OCM VOUT+ R G 4 6 R F 8
Understanding How They Work w/ Alternate Circuit Configurations Like Non-inverting Op Amp 2 Feedback Loops V IN V OCM V OUT- Differential feedback forces inputs to the same voltage V OUT+ Common mode Feedback forces V OUT- = -V OUT+- R G R F Non-inverting example: Like Inverting Op Amp R F For R F = 0 V Out+ = V IN Gain = 2 Inverting example: V IN R G V OCM V OUT- V OUT+ For R F = R G High input Z summing node V out- = -V IN Gain = 2 9
More About the V OCM Pin V OCM Pin separates our diff amps from other diff amp configurations Creates Best Available Balance @ High Frequencies Can be used with AC signal for Modulation as well as DC Reference Voltages Easy Level Shift From Ground Referenced Signals (+/-5V supplies) to Single +5V Supply Signals for ADCs Better Distortion in signal chain for +/-5V, than +5V Connect to the ADC reference or any other reference voltage 10
AD8131/2/8 vs. Dual Op Amp Configurations Compared to Dual Op Amp Configuration for Differential Driving: 2 Op Amps, G = +1 and G = -1 Output Dynamics are Different at High Frequencies Poor output balance; EMI emissions No Easy Way to Change Common Mode Output Level Distortion Products are Additive AD8131/2/8 even harmonics are Nulled by the Common Mode Feedback and Odd Harmonics are low by design 11
AD8131/2/8 vs. Transformers AD8131/2/8 are similar to Center-taped Transformers Differential or Single-ended In with Differential Out CM Output Adjustment AD8131/2/8 : Bandwidth to DC Does not require I/O impedance matching Can have signal power gain Smaller in size Lower cost than most transformers Has higher reliability V CM 12
Using the AD8138 in Active Filters Op amps have inverting and non-inverting inputs available. AD8138 inputs are both inverting Filter topologies must be inverting types. Note two feedback loops. V OCM - + V OUT DM 13
Filter Design Low-pass, High-pass and Band-pass Are Possible Butterworth, Bessel and Chebyshev Filters can be Realized in MF filters MF filters are 2nd Order (conjugate pole pairs) Higher order filters may be realized by stacking sections Multiple Feedback Filter topologies provide a DC path for the input bias current. Not acceptable SallenKey Low Pass Acceptable Multiple Feedback Low Pass + - - + 14
Differential Filter Characterization Low Pass, High pass and band-pass active filters were designed, built and tested As shown in the following slides, theoretical and actual results closely agree. AD8138 needs Small resistor values (10-47 Ohms) in series with the feedback circuitry to prevent oscillation at approximately 300 MHz. AD8132 does not Require a small resistor Feedback capacitance greater than a few pf may result in high frequency de-stabilization of the AD8132/8. 15
Ex.: 2 Pole Low Pass Schematic 2 Pole Low Pass Butterworth Anti-aliasing Filter, fc = 5 MHz 10 + In A 1.37k 1.37k 22p 332 +5V 100p + - In B 100p 1.37k 1.37k 332 22p - -5V 10 16
Differential Input to Single-ended Out Amps AD8129/30
AD8129/30 Receivers Active Feedback Topology, Like the AD830 High CMRR @ High Freq High Input Impedance CMRR Insensitive to Input Z Feedback network Independent of signal path Use as: Differential Receiver + & - Inputs have same Dynamic Response Difference Amp High Frequency InAmp Diff In AD8129/30 Vout V outref 18
AD8129/30 vs. Op Amp Configurations Compared to Single Op Amp Differential Amp Configuration for Receiver Poor CMRR Unbalanced Input Impedances Requires resistor matching for good CMRR Compared to 3 Op Amp Receiver Lots of parts and Design Time Extra Amps in Signal path lowers BW 19
AD8129/30 vs. Transformers AD8129/30 are similar to Transformers Differential In with Single-ended Out Output Reference Adjustment AD8129/30 : Bandwidth to DC Can have signal power gain Smaller in size Lower cost than most transformers Has higher reliability Diff In V OUT V OUT REF 20
For Use with High-speed Converters
ADCs Perform Better when Driven Differentially Especially as Frequency Increases THD [db] -30-40 -50-60 -70-80 -90 AD9240 (-6dBFS, 5V span) Single-ended Differential 0.1 0.2 0.5 1 2 5 10 20 Frequency [MHz] 22
AD8138 Driving an AD9224 12-bit 40 MSPS A/D on +5V Supply +5V +5 V 500.1.1 50Ω Source.1 500 525 V OCM 50 AD8138 50 V INB V INA A VSS A VDD D VDD AD9224 SENSE CML D VSS Digital Outputs 500-5V AD9224 Reference CML output drives V OCM to set optimum CM output Easy level shift using V OCM The AD8138 provides low-distortion drive on +5V or +/-5V Supplies 23
3V Circuit: AD8132 Driving an AD9203 10-bit 40 MSPS A/D +3V 499.1 +3V.1.1 1k 499 523 V OCM 49.9 49.9 20pF 20pF A IN- A IN+ A VDD D RVDD AD9203 A VSS D RVSS Digital Outputs 1k 499 AD8132 Provides +/-1V output swing on 3V supply with low distortion for low cost ADCs V OCM Level Shifts from Ground-referenced input Resistor and capacitor between Amp and ADC needed to filter Switchedinput current glitches 24
The AD8138 is the World s Best Amplifier for Driving High-speed ADCs > -80dB SFDR using the AD8138 to drive the AD9226 1Vp-p signal @ 20MHz SFDR over frequency 0-50 -20-60 [db] -40-60 SFDR-dB -70-80 +5V -80-100 -90 +/-5 V -120 5 10 15 20 25 30 Frequency [MHz] -100 0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+07 3.0E+07 3.5E+07 Frequency-Hz 25
Diff amps to Help Reduce Clock Jitter Some ADCs have Differential Clock Inputs to Minimize Ground Noise Effects on Jitter Ground Noise is only one source of jitter which decreases the performance of the fastest ADCs As Discussed before, With Differential Signals the Ground Noise becomes Common Mode AD8131/2/8 can be used to send the clock signal from its source into the ADC Isolating Analog and Digital Grounds Minimizing Radiated EMI 26
Buffered Differential Out for 12-16 bit High-speed DACs Virtual GND Reduces Effect of DAC s Nonlinear Output Impedance To Achieve Larger Output Power without having a large compliance voltage on the DAC Output When Level Shifting is Needed use V OCM 150W AD975X AD976X 0-20mA 20-0mA V OCM AD813x 150W 6V Diff Output 27
Differential to Singled-end Buffer for 12-16 bit High-speed DACs AD8129/30 can be used to Isolate the reactive load of the filter from the DAC output. Filter cap may be needed to reduce excessive slewrate on the amp input to improve amp settling To Achieve Larger Output Power without having a large compliance voltage on the DAC Output When Level Shifting is Needed use Ref input of the AD8129/30 AD975X AD976X 0-20mA 20-0mA 25W C F R G AD8130 R F Ref Input L-C LPF 28
For Driving and Receiving High-speed Signals
Differential Driver and Receiver V s+ 500W V s+ 500W AD8130 V s- V s- V fb V ocm Balanced Driver Minimizes EMI Generation High CMRR Receiver Minimizes EMI Pick-up 30
Cable Driving Challenge 0 db 1,000 ft. (300 m) CAT 5-UTP Want Transmitter plus Receiver Response to be Inverse of Cable Gain -20 db @ 10 MHz -60 db @ 100 MHz 100kHz 1MHz 10MHz Frequency 100MHz 31
Drive / Receive Requirements Driver Balance needed to minimize radiated EMI Simple to use, no Z matching required High BW to transmit boosted signal Receiver CMRR needed to reject CM Noise Feedback network independent of receive section High BW for equalization boost [Output Balance Error dbc] -20-30 -40-50 -60-70 AD8138 Output Balance Vs Frequency +/-5V +5V 1 10 100 1000 Frequency CMRR (db) AD8130 Common Mode Rejection vs Frequency (Vs = +/-5V, Vcm = 1Vp-p) -20-30 -40-50 -60-70 -80-90 -100-110 -120 0.01 0.1 1 10 100 Frequency (MHz) 32
Receive-Side Equalization AD8129/30 Equalized Signal Out Receiver Line equalization 40dB or more of gain can be achieved at high frequency Feedback network zeros set the gain AD8129 has more GBWP and Lower noise Rf Equalizing Network Gain HF Boost Frequency 33
Drive Side High Frequency Boost +5V 10pF 249 249 V OCM 499 49.9 49.9 Twisted -pair Cable 100 24.9 10pF 499-5V Integrator on input adds zero to boost signal @ high frequency For Equalization when Driving Long Cables Gain Limited by Output swing capability 34
AD8132 Makes Simple Very High Speed Full-wave Rectifier +5V 348 348 V OCM 49.9 348 348 HP2835 (Shottky) -5V Useful for measuring RMS of AC Signals Operates to greater than 300 MHz 50 FW Rectified Output 35
ADI Multi-Purpose Differential Amp Family Part # AD8131 AD8138 AD8132 AD8129 AD8130 Differential-to-Differential Driver Differential-to-Single Ended Receiver Features Fixed Gain=2x Adjustable Gain / Feedback 10x stable 1x stable Bandwidth 400MHz 310MHz 350MHz 200MHz 270MHz Slew Rate 2000V/µs 1150V/µs 1200V/µs 1060V/µs 1090V/µs Position Line Driver Best ADC Driver Low Cost Gen Purp Diff-to-S.E. converter Diff-to-S.E. converter 36
High-Speed Amplifiers (HSA)
Fast FETs The NEW Standard for JFET Amplifiers Very Easy to Use Negligible I bias and I noise R-R output Wide supply range Low Supply Current Low Price 38
Fast FETs Low-Cost High-Speed AD8033/4 AD8033/4 75MHz Bandwidth 80V/µs Slew Rate 3.2mA/Amp Typical Supply Current Rail-to-Rail output Wide Supply Range 5-24V Very Low Pricing $1.19 @ 1K - AD8033 (Single) $1.59 @ 1K - AD8034 (Dual) Part Status Final Silicon Release Qtr AD8033 (3Q02) AD8034 (2Q02) SOT23-8 39
Fast FETs High-Performance High-Speed AD8065/6 AD8065/66 140MHz Bandwidth 160V/µs Slew Rate 7 nv/vhz Noise 6.5mA/Amp Typical supply current Rail-to-Rail output Low offset voltage and drift Wide Supply Range 5-24V Price @ 1K $1.59 - AD8065 (Single) $2.29 - AD8066 (Dual) Part Status Final Silicon Release Qtr AD8065 (2Q02) AD8066 (3Q02) 40
Ultra Low-Distortion and Noise Amplifier AD8007/8 Extremely Low SFDR -96dB@5MHz -86dB@20MHz -55dB@70MHz Low Noise 2.6 nv/vhz 22 pa/vhz High Speed 600MHz Bandwidth 1000V/µs Slew Rate Low Power 9mA/Amp Typical supply current Pricing @ 1k $1.19 - AD8007 $1.99 - AD8008 Part Status Final Silicon Release Qtr AD8007 (3Q02) AD8008 (1Q03) 41
Low-Power High-Speed Amplifier AD8038/9 Low Power 1.1mA/Amp Typical supply current High Speed 315MHz Bandwidth 425V/µs Slew Rate Low Noise 250pA/vHz 7nV/vHz Low SFDR -86dB @ 1MHz -77dB @ 5MHz Price @ 1K $0.85 - AD8038 $1.20 - AD8039 Part Status Final Silicon Release Qtr AD8038 (2Q02) AD8039 (1Q02) 42
Summary of New Products Generic Description Samples Release AD8033 Fast FETs Low-Cost High-Speed (single) 1Q02 3Q02 AD8034 Fast FETs Low-Cost High-Speed (dual) Now 2Q02 AD8065 Fast FETs High-Performance High-Speed (single) Now 2Q02 AD8066 Fast FETs High-Performance High-Speed (dual) 1Q02 3Q02 AD8007 Ultra Low-Distortion and Noise (single) Now 3Q02 AD8008 Ultra Low-Distortion and Noise (dual) 1Q02 1Q03 AD8038 Low-Power High-Speed (single) 1Q02 2Q01 AD8039 Low-Power High-Speed (dual) Now 1Q01 43