I am using the ADuCM360 ADC to measure a signal with 1.6V common mode using PGA Gain = 2. The differential signal is typically small and fits within the ±500mV PGA input range, however, it is driven by an op-amp with ±5V rails. Under certain conditions the input to the ADC can swing to ±4.6V but I have included a 100Ω series resistor to limit current into the ADC's protection diodes.
Question: How much current can the ADC protection diodes divert? I do not see this information in the datasheet or manual, only an absolute max voltage of -0.3V to +3.96V. Are there other side effects to providing an ADC input this far over range?
It seems that the input voltage in your application will exceed the max voltage. Though you add a 100 series resistor ahead the ADC input pins, I don't think it's a reliable method to resolve the issue. Even the protection diodes can provide several mA current.
Could you please reduce the Amplifier gain and use the PGA gain inside ADuCM360 and make the input voltage match the ADC requirements.
The amplifier gain is correct for my signal dynamic range (±500mV out), the problem is that the amplifier voltage rails exceed the safe input range of the ADC so in certain conditions the output can swing to ±4V. Unfortunately I already have a board without the required clamping so I am trying to find the easiest way to implement a fix. Perhaps I can just increase the 100Ω series resistance to keep the current acceptably low when the signal swings off scale, but maybe I will need to squeeze in clamp diodes.
The datasheet says the unbuffered input current is 500nA/V which gives an input impedance of 2MΩ. But in the buffered mode it seems to be more like 1GΩ, so a 1kΩ source impedance still puts the error at the 1ppm (20bit) level which is better than my ADC performance anyway.
If you configure the PGA value more than 1, actually the PGA can be seen as a buffer. So the input current will be around 1nA no matter the buffer is on or off. Please note this point.
Adding a clamp diode perhaps is a workaround, but please pay attention to the leakage current.