Attenuating a signal from a eddy current sensor for an ADC

Question

I have a eddy current sensor, in a 7200 Proximity Transducer System, with a voltage output of -18V to -24V. My ADC has a max input range of +10V. I need a way to step the output signal down so I can feed it to my DAQ and not damage it. I'm looking for an industrial solution, in the same way we have strain gauge signal conditioners/amplifiers, does something like this exist for an attenuator?

Answer 1

To make this analog voltage compatible with your DAQ input requires a simple transformation: polarity conversion, ie: convert an analog voltage that is negative wrt GND (0 to -24V) to an analog voltage that is positive wrt GND (0 to +10V).

One option is to buy an off-the-shelf product, known as a "signal conditioner", these are available from several suppliers, such as Phoenix Contact, model "MINI MCR-2-U-U-PT", link below:

https://www.phoenixcontact.com/en-au/products/signal-conditioner-mini-mcr-2-u-u-pt-2902043

The benefit of purchasing a signal conditioner is that they may provide other functions that you may require, such as: galvanic isolation, and intrinsic safety. They also have all the approvals you may need for legal reasons (safety, EMC, etc).

The other option is to "roll your own"; functionally what you require is quite simple, however, the complexity will occur if you require any of the other features mentioned. If you don't need those other features, then this could be as simple as an inverting amplifier, as per the circuit below:

^{simulate this circuit – Schematic created using CircuitLab}

U1 is the sensor; the op-amp shown here is just a suggestion (it is the default provided by the schematic drawing tool) and may not be the most appropriate for your application.

Select R1 & R2 to do the following:
(a) load the sensor correctly, and
(b) convert the negative signal to the correct scaled positive signal.

The values shown here will convert -20V (negative) at the sensor output to +10V (positive) at the op-amp output.

V1 can be changed to suit the op-amp and whatever may be available. +12V is probably close to the minimum value, this allows op-amp output to reach up to +10V; but it could be changed a higher voltage, eg: +24V.

Appendix:
The sensor you specified in the comments is a proximity sensor based on the well-known eddy current effect. The basic principle of which is: an RF transmitter creates a field that interacts with the target; the distance to target is encoded by changes in the electrical parameters of either the (a) RF transmitter or (b) RF receiver or (c) a combination of both (a) and (b).

Yes, the output signal is negative, as evidenced by the snippets below:

Above image: taken from datasheet at link provided by OP:
https://www.protectivesupplies.com/datasheets/8_7200_5_8mm_datasheet_1990-1991_(1).pdf

Above image: taken from datasheet at link:
https://dam.bakerhughes.com/m/b5705b96741a2ed/original/bently-nevada-proximity-probe-system-reliability-benefits-en-pdf.pdf

The reason the output signal is negative wrt GND rather than positive may have to do with preventing galvanic corrosion.