I have an IEPE accelerometer (this one) that I'd like to connect to an audio recorder. IEPE accelerometers have two cables (signal and ground) and require 18-30V DC applied to the signal cable to power the internal electronics. The signal (±5 V or so) hence has a DC bias that has to be removed before passing it on to a recorder. Think of the accelerometer as a contact microphone providing signals between 0 and 20 kHz.
When I first received the accelerometer, someone with experience using them provided a circuit diagram to use; here's my recreation:
As I understand it, capacitor C1 and resistor R2 act as a high-pass filter and remove the DC bias; R1 limits the current to the accelerometer, connected at the BNC inputs. (The accelerometer manual specifies a constant-current diode instead, between 2-20 mA, but I don't yet have one.)
Initially I powered this with three 9V batteries in series and it worked very well. But I wanted to make a portable device that only used one 9V battery, so I got an adjustable boost-buck converter to boost it to 20 V, assuming that the switching noise would be above the audible range. Unfortunately I was wrong, and there's still some quiet-but-noticeable high-frequency noise at around 7 kHz.
My question: How do I filter the noise? I could use a low-pass filter after the power supply, by putting a capacitor between signal and ground, but would that also distort the accelerometer input in some way? Or would something like a T filter remove the noise without affecting the input from the accelerometer?
(If it's relevant, the accelerometer output impedance is specified to be ≤300 ohms; full specs are available here.)