Next step in frequency response beyond AD8429

Hello,

I'm trying to measure a ~50mV differential voltage output from a Wheatstone bridge. The common mode voltage is around 7.5V due to the supply voltage of 15V. I am looking for a gain of 20-30 in order to obtain good signal strength for an Oscilloscope measurement, without sacrificing bandwidth. My application needs a flat response out to about 1-3 MHz, whereas the high bandwidth monolithic instrumentation amplifiers, such as http://www.analog.com/static/imported-files/data_sheets/AD8429.pdf, have a 3dB bandwidth near 3 MHz for gains of 10+ (not flat out to 1-3 MHz region).

My question is, what is preventing me from using an ultra-fast op-amp, such as http://www.analog.com/en/high-speed-op-amps/current-feedback-amplifiers/ad8003/products/product.html, and arranging it in an instrumentation amplifier topology by adding trimmed resistors. The AD8003 even includes 3 op-amps in the package, which would be nice for in-amp design. I have read that a good rule of thumb is to have your measuring device capable of measuring 20-30x the frequency you wish to measure in order to ensure good measurement capability.

I know there are other factors at play: picking up ambient high frequency signals, PCB layout and shielding layers to ensure correct functionality, etc. and it does seem that the AD8003 might be overkill, but I know there must be a basic limitation that I am overlooking and am essentially asking you all to point out my oversight.

Or, if you know of a good high-speed monolithic instrumentation amplifier or different amplifier topology that I should be pursuing, that would be great too.

Thanks!

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  • Hi Bryan,

    I support the AD8003 as well as our other high-speed current feedback amplifiers (CFAs).  I have to disagree with the comment that CFAs cannotbe used in the traditional 3-amplifier in-amp configuration.  We offer a number of DSL drivers that are basically dual CFAs that are generally configured in the in "bridge" configuration used on the in-amp input.  The "back end" four-resistor difference amplifier configuration used in in-amps can also be constructed with CFAs.  The bottom line is that CFAs can be used to construct a 3-amp in-amp, i.e., they can be made to be stable, but they do have drawbacks that must be considered.

    First of all, CFA stability is determined by the feedback resistor (Rf) -- it must always be a resistor (no feedback capacitors, etc...).  Typical feedback resistor values are a few hundred ohms.  CFAs use an error current as feedback, and high transimpedance, Z(s), from input to output rather than voltage gain, A(s), as is used in voltage feedback amplifiers (VFAs).  CFA loop gain is equal to Z(s)/Rf, so distortion performance, bandwidth, closed-loop output impedance, etc... are all determined by Rf.  As Rf decreases, loop gain increases, as does bandwidth, distortion performance, etc..., but stability (phase margin) decreases.  Generally, Rf is chosen for phase margin between 45 and 60 degrees.  The data sheets indicate recommended Rf values.

    CFAs are designed for low-impedance, high-speed applications, often at the expense of DC performance.  Things like input bias current, offset voltage, drifts, 1/f noise, etc... are not as good as they are for FET-input amps, precision amps, etc...  This, however, does not mean that they cannot be used in an in-amp configuration.  If your application can tolerate the high input bias current (7uA for the AD8003) and reduced DC performance, CFAs may be a viable option.  Please let me know if you have more questions pertaining to the AD8003.

    Bet regards,

    --Jonathan

Reply
  • Hi Bryan,

    I support the AD8003 as well as our other high-speed current feedback amplifiers (CFAs).  I have to disagree with the comment that CFAs cannotbe used in the traditional 3-amplifier in-amp configuration.  We offer a number of DSL drivers that are basically dual CFAs that are generally configured in the in "bridge" configuration used on the in-amp input.  The "back end" four-resistor difference amplifier configuration used in in-amps can also be constructed with CFAs.  The bottom line is that CFAs can be used to construct a 3-amp in-amp, i.e., they can be made to be stable, but they do have drawbacks that must be considered.

    First of all, CFA stability is determined by the feedback resistor (Rf) -- it must always be a resistor (no feedback capacitors, etc...).  Typical feedback resistor values are a few hundred ohms.  CFAs use an error current as feedback, and high transimpedance, Z(s), from input to output rather than voltage gain, A(s), as is used in voltage feedback amplifiers (VFAs).  CFA loop gain is equal to Z(s)/Rf, so distortion performance, bandwidth, closed-loop output impedance, etc... are all determined by Rf.  As Rf decreases, loop gain increases, as does bandwidth, distortion performance, etc..., but stability (phase margin) decreases.  Generally, Rf is chosen for phase margin between 45 and 60 degrees.  The data sheets indicate recommended Rf values.

    CFAs are designed for low-impedance, high-speed applications, often at the expense of DC performance.  Things like input bias current, offset voltage, drifts, 1/f noise, etc... are not as good as they are for FET-input amps, precision amps, etc...  This, however, does not mean that they cannot be used in an in-amp configuration.  If your application can tolerate the high input bias current (7uA for the AD8003) and reduced DC performance, CFAs may be a viable option.  Please let me know if you have more questions pertaining to the AD8003.

    Bet regards,

    --Jonathan

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