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how to improve the CMRR of opamp?

1) Kindly provide any other opamp having higher CMRR,  but with similar specification of ADA4895.

2) Kindly provide methods to improve CMRR of opamp apart from resistor matching.

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  • Hi rmurthy720,

    The guaranteed DC CMRR of 100dB for ADA4895 is hard to beat for most high frequency devices I searched for. However, I've shown a list of comparable ADI devices you could look through:

    Part # Feedback Type CMRR (min) Temp Range # of Amps GBP (typ) Slew Rate (typ) Vos (max) VNoise Density (typ) Ibias (max) Iq/Amp (typ) Vs span (min) Vs span (max) Price (1000+) Associated Model Package Area
        (dB)     (Hz) (V/us) (V) (V/rtHz) (A) (A) (V) (V) ($ US)   (mm²)
    LTC6229 Voltage 100 -40 to 125°C 2 890M 500 95µ 880p 2.5µ 16m 2.8 11.75 $2.37 (LTC6229IDD#PBF) 9mm² (10-Lead DFN (3mm x 3mm w/ EP)) (LTC6229HDD#PBF)
    LTC6228 Voltage 100 -40 to 125°C 1 890M 500 95µ 880p 2.5µ 16m 2.8 11.75 $1.58 (LTC6228IDC#TRPBF) 4mm² (6-Lead DFN (2mm x 2mm w/ EP)) (LTC6228HDC#TRPBF)
    LT1222 Voltage 100 -40 to 85°C 1 500M 200 300µ 3n 300n 8m 5 36 $4.25 (LT1222CN8#PBF) 31.00979mm² (8-Lead SOIC (Narrow 0.15 Inch)) (LT1222CS8#PBF)
    ADA4895-2 Voltage 100 -40 to 125°C 2 1.5G 943 350µ 1n 16µ 3m 3 10 $3.21 (ADA4895-2ARMZ) 15.965mm² (10-Lead MSOP) (ADA4895-2ARMZ)
    ADA4895-1 Voltage 100 -40 to 125°C 1 1.5G 943 350µ 1n 16µ 3m 3 10 $1.89 (ADA4895-1ARJZ-R7) 9mm² (6-Lead SOT-23) (ADA4895-1ARJZ-R7)
    AD829 Voltage 100 -55 to 125°C 1 750M 230 1m 1.7n 5.3m 9 36 $3.65 (AD829ARZ) 31mm² (8-Lead SOIC) (AD829JRZ-REEL)
    AD8099 Voltage 98 -40 to 125°C 1 3.8G 470 500µ 950p 13µ 15m 5 12 $2.00 (AD8099ACPZ-REEL) 9mm² (8-Lead LFCSP (3mm x 3mm w/ EP)) (AD8099ACPZ-REEL7)
    LT6230-10 Voltage 95 -40 to 85°C 1 1.45G 250 500µ 1.1n 10µ 3.3m 3 12.6 $1.50 (LT6230CS6-10#TRPBF) 8.12mm² (6-Lead TSOT-23) (LT6230CS6-10#TRMPBF)
    LT1226 Voltage 94 0 to 70°C 1 1G 400 1m 2.6n 7m 5 36 $3.28 (LT1226CN8#PBF) 31.00979mm² (8-Lead SOIC (Narrow 0.15 Inch)) (LT1226CS8#PBF)
    AD8021 Voltage 86 -40 to 85°C 1 1G 130 1m 2.1n 11.3µ 7.8m 4.5 24 $1.31 (AD8021ARMZ) 31mm² (8-Lead SOIC) (AD8021ARZ-REEL7)
    LTC6254 Voltage 85 -40 to 125°C 4 720M 280 350µ 2.75n 3.3m 2.5 5.25 $3.55 (LTC6254CMS#PBF) 19.7911mm² (16-Lead MSOP) (LTC6254CMS#TRPBF)

    To improve CMRR: If there is anything that can be done to reduce the CM voltage variation that the device experiences, that would of course reduce the error voltage.

    Also, I've not tested this but if you were to introduce a diff pair in front of an Op Amp (in a composite amplifier configuration), and used precision / tracking / matched resistors) this would boost the differential gain and should reduce CMRR. However, something tells me that you give up many things (e.g. offset error, bandwidth, noise, etc.) when you do something like this as a penalty. So, I'm not sure how practical this approach may be, but thought I mention it anyway.

    Regards,

    Hooman

Reply
  • Hi rmurthy720,

    The guaranteed DC CMRR of 100dB for ADA4895 is hard to beat for most high frequency devices I searched for. However, I've shown a list of comparable ADI devices you could look through:

    Part # Feedback Type CMRR (min) Temp Range # of Amps GBP (typ) Slew Rate (typ) Vos (max) VNoise Density (typ) Ibias (max) Iq/Amp (typ) Vs span (min) Vs span (max) Price (1000+) Associated Model Package Area
        (dB)     (Hz) (V/us) (V) (V/rtHz) (A) (A) (V) (V) ($ US)   (mm²)
    LTC6229 Voltage 100 -40 to 125°C 2 890M 500 95µ 880p 2.5µ 16m 2.8 11.75 $2.37 (LTC6229IDD#PBF) 9mm² (10-Lead DFN (3mm x 3mm w/ EP)) (LTC6229HDD#PBF)
    LTC6228 Voltage 100 -40 to 125°C 1 890M 500 95µ 880p 2.5µ 16m 2.8 11.75 $1.58 (LTC6228IDC#TRPBF) 4mm² (6-Lead DFN (2mm x 2mm w/ EP)) (LTC6228HDC#TRPBF)
    LT1222 Voltage 100 -40 to 85°C 1 500M 200 300µ 3n 300n 8m 5 36 $4.25 (LT1222CN8#PBF) 31.00979mm² (8-Lead SOIC (Narrow 0.15 Inch)) (LT1222CS8#PBF)
    ADA4895-2 Voltage 100 -40 to 125°C 2 1.5G 943 350µ 1n 16µ 3m 3 10 $3.21 (ADA4895-2ARMZ) 15.965mm² (10-Lead MSOP) (ADA4895-2ARMZ)
    ADA4895-1 Voltage 100 -40 to 125°C 1 1.5G 943 350µ 1n 16µ 3m 3 10 $1.89 (ADA4895-1ARJZ-R7) 9mm² (6-Lead SOT-23) (ADA4895-1ARJZ-R7)
    AD829 Voltage 100 -55 to 125°C 1 750M 230 1m 1.7n 5.3m 9 36 $3.65 (AD829ARZ) 31mm² (8-Lead SOIC) (AD829JRZ-REEL)
    AD8099 Voltage 98 -40 to 125°C 1 3.8G 470 500µ 950p 13µ 15m 5 12 $2.00 (AD8099ACPZ-REEL) 9mm² (8-Lead LFCSP (3mm x 3mm w/ EP)) (AD8099ACPZ-REEL7)
    LT6230-10 Voltage 95 -40 to 85°C 1 1.45G 250 500µ 1.1n 10µ 3.3m 3 12.6 $1.50 (LT6230CS6-10#TRPBF) 8.12mm² (6-Lead TSOT-23) (LT6230CS6-10#TRMPBF)
    LT1226 Voltage 94 0 to 70°C 1 1G 400 1m 2.6n 7m 5 36 $3.28 (LT1226CN8#PBF) 31.00979mm² (8-Lead SOIC (Narrow 0.15 Inch)) (LT1226CS8#PBF)
    AD8021 Voltage 86 -40 to 85°C 1 1G 130 1m 2.1n 11.3µ 7.8m 4.5 24 $1.31 (AD8021ARMZ) 31mm² (8-Lead SOIC) (AD8021ARZ-REEL7)
    LTC6254 Voltage 85 -40 to 125°C 4 720M 280 350µ 2.75n 3.3m 2.5 5.25 $3.55 (LTC6254CMS#PBF) 19.7911mm² (16-Lead MSOP) (LTC6254CMS#TRPBF)

    To improve CMRR: If there is anything that can be done to reduce the CM voltage variation that the device experiences, that would of course reduce the error voltage.

    Also, I've not tested this but if you were to introduce a diff pair in front of an Op Amp (in a composite amplifier configuration), and used precision / tracking / matched resistors) this would boost the differential gain and should reduce CMRR. However, something tells me that you give up many things (e.g. offset error, bandwidth, noise, etc.) when you do something like this as a penalty. So, I'm not sure how practical this approach may be, but thought I mention it anyway.

    Regards,

    Hooman

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