Vibration Measurements 

Using the ACH-01 Accelerometer

Some years back I bought a couple of the Measurement Specialties ACH-01 accelerometer (datasheet here). This handy little unit has been used for many applications, including motional feedback in loudspeakers (servo systems). It has a three-wire interface: ground, signal (source of the built-in J-FET) and supply (drain of the J-FET). Basically the same as some electret microphones. This requires a preamp, and the datasheet shows a circuit with a large resistor from the J-FET source to -Vdd, and capacitor coupling to a FET-input op-amp. 

But as I was going to use this with measurement SW like Arta, I would connect it to a sound card, and since most external sound cards have a microphone preamps with phantom power, I decided to just build a phantom-powered buffer for it. The circuit is shown below.

The ACH-01 should draw about 80-90µA. To not compromise linearity, a J-FET constant current source (CCS) is used (Q1). The ACH screen is actually not connected to DC ground but to a virtual ground at half the supply voltage, to create greater headroom for the internal J-FET. But it is AC grounded thanks to C2, so the shielding is effective. But this means that you have to use a connector that is isolated from ground. I used an isolated phone jack socket. 

The signal is buffered by Q2, which also has a CCS in the source lead. You can use other types of CCS here, like a J-FET, transistor + LED or the ring of two that I used, which gives a very well defined current. 

Phantom power is taken from pins 2 and 3 in the XLR through R7 and R8, and decoupled and stabilized by C3 and DZ1. 

My implementation is shown below. It was built into a small die-cast box, making it a robust little unit. 

Now the accelerometer itself can be attached to anything you want to measure, and will behave like a phantom-powered microphone to your soundcard. Other applications include using it as a pickup for acoustic guitar, putting it on the bass drum, and any other application where you need to pick up vibrations.

Below is an example measurement from the back wall of an unbraced enclosure. The resonance frequencies can be easily picked out.