Ask the experts

Programming the gain on a model 133 and 136 signal conditioner

Many users sometimes become confused when attempting to configure a model 133 or 136 signal conditioner, particularly when trying to program the “gain” of the amplifier. However, armed with a few simple rules and some basic knowledge of these popular signal conditioners, it becomes really quite simple.

The most basic rule, but perhaps the most misunderstood, is that the user cannot actually set the gain of the amplifier directly. Once the user configures a couple of parameters, the amplifier itself sets the appropriate gain to meet the user’s configured requirements.

The second rule, also frequently misunderstood, is that these amplifiers do not concern themselves with mechanical engineering units (i.e. g, psi, lbs, etc.). The only units the model 133 uses is pC (pico-coulombs) when configured as a charge amplifier, and mV when configured as an ISOTRON (IEPE type) amplifier. The 136 only uses mV. So, for example, when programming the input sensitivity for a charge accelerometer, the 133 will display pC/EU (where EU is engineering unit). When programming the output scale factor, both amplifiers only use mV/EU.

At first, it may seem strange that the 133/136 does not allow the user to set the gain directly. The advantage of this, however, will become clear with some explanation. What the user does set is the input sensitivity from the input transducer, and the desired output scale factor. The amplifier then determines what gain is necessary to meet these parameters. This frees the user from having to calculate gains, and offers a tremendous amount of flexibility in the output scale factor that can be set.

So, how to begin? Step 1 is to determine the sensitivity of the accelerometer being used. This can usually be found on the calibration certificate that came with the sensor. If not, contact the manufacturer for further information. Next, program this value into the 133/136. This step is usually straightforward.

The next step is a bit more complicated. Rather than programming gain, the user will program the output scale factor. This is the value a downstream (connected to the output of the 133/136) data acquisition (DAQ) system will use to process the output signal.

What is meant by “output scale factor?” This is the amplifier’s electrical signal output (in mV) to mechanical parameter input (g, psi, lbs, etc.) ratio. Output scale factor can be thought of as simply the accelerometer’s sensitivity, except that it may be gained up. For example, the user may have a 10 mV/g ISOTRON accelerometer on the input, and have the output scale factor also set to 10 mV/g. In this case, the amplifier is only using a gain of 1. But this doesn’t take advantage of the 133/136 signal conditioner’s capabilities. For example, using that same 10 mV/g accelerometer, the user could set the output scale factor to 100 mV/g, causing the amplifier to use a gain of 10. A better and more realistic example is this: the input accelerometer has a sensitivity of 9.48 mV/g. The user can simply set the output scale factor to 100 mV/g (or 10 mV/g, or whatever is convenient) and the amplifier figures out the correct gain to accomplish that. The only restriction is that the 133/136 has a maximum gain limitation of 1000 (note that gain can be calculated by dividing the output scale factor by the input sensitivity).

Another way to approach programming output scale factor is to consider what the output full scale voltage may need to be from the amplifier. Some DAQ systems may even require this. For example, say that the DAQ system needs 1 V full scale. The user in this example knows to expect no more than 2 g maximum in their measurement. To take full advantage of the amplifier’s resolution, the user can program the output scaling such that 1 V will represent 2 g. What would the output scale factor be in this case? Divide 1000 mV (remember, the 133/136 only uses mV) by 2 g, or 1000 mV/2 g, or 500 mV/g.