I want to design a circuit, that allows me to measure the voltage across a piezoelectric transducer.
The transducer itself has an impedance |Z| of 500 Ohms up to ~20 kOhms.
The voltage to measure ranges from ~5Vpp up to max. 400Vpp at frequencies from 20kHz to 50kHz.
On my planned PCB, I will use a 16-Bit ADC with a input voltage range of 0V to 5V. Hence, I will need to convert this high voltage signal down to the measurement range of the ADC.
As you can see on the above schematic, I am planning to use a simple voltage-divider that scales the input voltage down by a factor of 200 and has a high input impedance of 10MOhms (connected in parallel to the transducer).
The downscaled signal is then picked up and amplified by a factor of 2 with the instrumentation amplifier INA819, which has a high input impedance (GOhms), such that loading of the divider output is kept at a minimum.
The total gain of this circuit is then 1/100. The common mode input voltage range of the instrumentation amplifier is respected and at least in TINA TI simulation, everything seems to work fine and even the simulated measurement noise on the ADC is with ~10mVRMS (of the sensed voltage) pretty good.
However, I am afraid that especially the high resistances on my potential divider might cause some problems on the real circuit due to parasitic effects.
As an example:
The instrumentation amplifier datasheet provides data about the input capacitance, which is 1pF (differential) and 4pF (common-mode).
Could it be, that these capacitances in combination with the high input resistance act as a low pass filter for my signal? And if yes, how can I calculate or simulate this?
Of course, there will be other parasitic capacitances on my circuit because of the traces from the divider to the amplifier. To keep these at a minimum, I want to make the trace-widths as thin as the PCB manufacturer's standard-class allows it. Could it also be of advantage to remove any ground-planes below the dividers and the amp's input?
Are there other critical aspects that I miss?
Would you consider this circuit "good practice" for my kind of application or are there better methods available?