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
You have pointed out many valid points. Instrumentation amplifiers like AD8429 don't lend themselves very well to build fast and precise PGIAs because of the problems you mention in your post, being the parasitics added to the RG pins one of the most problematic aspects. You can find more details here:
If you feel that AD8253 is not suitable because of its input noise, have you considered rolling up your own PGIA? Do you have enough space on your board? You could do it with a small low-noise dual Op Amp (like AD8599 for precision or ADA4898-2 if you need faster but bigger) followed by difference amplifier like AD8271. The difference amplifier would give you excellent CMRR at low gains and save you the resistors for the subtractor. An advantage is that once you have access to the inputs of the Op Amps, you don't need to worry so much about the channel resistance because it does not show up in series with your gain resistors, and can increase it to minimize input capacitance. And then, of course, you can compensate it to avoid gain peaking or instability. Of course, you can increase complexity at the expense of cost and board area, but I gather you don't have much of the latter.
Let me know what you think...