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AD8429 gain switching

Question asked by bdolf on Feb 12, 2015
Latest reply on Feb 20, 2015 by bdolf

For a data acquisition frontend we need an instrumentation amplifier with wide bandwidth, low distortion, and low noise. We are considering the AD8429. The frontend needs to capture signals with +/-10V, 1V, and 100mV full-scale range, requiring to switch the AD8429 gain setting resistor.

PCB space contraints do not allow to use relays as switches, so we are considering analog switches as shown in CN0146. Unfortunately the suitable analog switches with low Rds_on and low delta-Rds do have large input and output capacitances (e.g. ADG1402: Cd_off = 41pF, Cs_off = 36pF). If such a switch is connected to the RG-terminals these capacitances become part of the feedback network and reduce the feedback for the higher frequencies, resulting in a frequency-dependent rise of gain. In case of the AD8429 simulation and calculation show an 18dB peak at ~9MHz for gain=1, and a 5dB peak at gain=10. We are afraid these peaks, esp. at gain=1, will cause problems with increased HF-noise, RFI immunity and stability. Since we cannot access the internal feedback resistors, we see no chance to compensate the parasitic capacitance of the analog switches.

We cannot use an instrumentation amp with lower GBW (which will "mask" the gain rise) because this would significantly increase distortion on higher input frequencies. Also the AD8253 with built-in gain switches is not a suitable alternative because of its higher input noise.

Has anyone encountered (and hopefully solved) a similar problem?

Many thanks, Bdolf