Accelerometer output impedence

[PeterDove]

"Both PIC and AVR datasheets specify that for A-D conversion to work properly, the connected device must have an output impedance under 10kΩ. Unfortunately, Analog Devices' analog accelerometers have an output impedance of 32kΩ."

This is a quote from a site I read, I have been wanting to use Analog accelerometer devices. Can anyone explain what on earth this means? what is output imedence? What must I do to deal with it and has anyone had experience in this.

Peter

 

[dknguyen]

Well you know the resistor is there to act with an external capacitor to ground to be a low-pass filter right?

Impedance is like resistance, but more general. You see, resistance does not change with frequency. Impedances is similar to a frequency dependent resistance. Different frequency signals (and frequency components making up a signal) see the impedance at different values. For example, a signal's higher frequency components might see less "resistance" than the lower frequency components of the signal)..

INput/output impedance (or resistance if it does not vary with frequency) is the impedance that is seen when measuring across the input/output terminals. Something that has a very low resistance input means that you must drive LOT of current through it to get a certain voltage signal to appear. This same current must pass through the output resistance of device A before it goes into device B where it encounters the input resistance. If output resistance is too large, this current will cause too large a voltage drop across the output resistance before it travels to the input resistance, resulting in loss of signal integrity. it's like a voltage divider. The smaller one resistor is, the more of the input voltage appears across the other resistor. In this case, you want as much of the signal voltage as possible to appear across the "output resistor", and as little to appear as possible across the "input resistor."

Basically- just use an op-amp buffer between the accel and ADC. This basically has a high impedance input so the output doesnt need a lot of current to create a voltage (less current means less voltage drop across the output resistance of the accel). ANd the buffer output is low impedance and can drive lots of current to overcome the input impedance of the ADC.

 

[PeterDove]

Basically- just use an op-amp buffer between the accel and ADC. This basically has a high impedance input so the output doesnt need a lot of current to create a voltage (less current means less voltage drop across the output resistance of the accel). ANd the buffer output is low impedance and can drive lots of current to overcome the input impedance of the ADC.

Thanks for the explanation. Do you have any pionters to a circuit showing how to set up the opamps for this purpose?

Peter

 

[Nigel Goodwin]

Thanks for the explanation. Do you have any pionters to a circuit showing how to set up the opamps for this purpose?

You could check my A2D PIC tutorial!.

 

[donniedj]

That 10K is a recommendation but not an absolute. As the load impedance goes beyond the recommendation the connection turns into a voltage divider which just worsens any actual external voltage division to the PIC pin and also the sampling capacitor take longer to charge to True voltage hence fast samples end up attenuating the True voltage.

I usually have very high impedance from 50K to 1M ohms and refuse to add ancillary parts to buffer the signal. So what I do is offset the voltage in the code. Measure the volts at the PIC pin with a good scope and adjust the value in the code to match. Regardless, calibration is needed anyway whether it be in the form of code or with external parts. Using an op-amp buffer is a physical means of calibration.