We are driving the analog input of the ADS8328 with an instrumentation amplifier being powered by +/- 7VDC. The SDC is powered by a separate 5V supply. There is also a passive LPF on the input consisting of a 10K resistor and a 10nF cap. We found our response time to be too low for our liking so we changed the 10K to a 1K, which improved our response time dramatically.
However, what happens when the input amp powers up before the ADC, it could inject anywhere from -3V to +7.5V into the ADC input (bad). Makes us wonder, Is the analog input always high impedance or just when powered on.
Unless I am missing something, the data sheet doesn't really say too much about the input impedance, except for when the ADC is in hold mode (1Gohm).
Can we just change the Input filter resistor to limit the current so that the input voltage to the ADC doesn't exceed its limit? We realize that it would change our response times, but we might be able to live with that.
If increasing the resistor value won't work, is there anything we can do to limit the current into the ADC so we don't blow the input? Because of the maturity of the design, we prefer to change the circuit as little as possible.
Thanks,
Glenn
