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New Instrumentation Amplifier Makes Sensing Easy

The Intersil ISL2853x/63x series IAs are pretty cool devices. These are a series of instrumentation amplifiers based on the usual three-op-amp configuration. They are built on a low-voltage process, so they can be powered from +5V or ±2.5V. With bipolar supplies, they can handle bipolar signals across the corresponding range.

The devices are based on one of Intersil's chopper-stabilized op-amps. That means virtually no drift of input offset voltage (which is already very low). And it's pretty low noise. The internal resistors are sufficiently matched to give very good CMRR. They are characterized for up to 125°C, meaning that they are suitable for industrial and automotive applications.

The output of the ISL2853x is single-ended and can drive an ADC with a single-ended input. Note the uncommitted op-amp that can be used for various functions.

The output of the ISL2863x is differential and is intended to drive a differential-in ADC.

The 'x' in those part numbers is 3, 4, or 5. It indicates the set of gains you get when setting the gain bits (G0 and G1). The terminals on the IC marked as VA+ and VA- can be used for a separate amplifier to derive a common mode voltage for the bridge circuit. This is useful to indicate the status of the bridge — whether it is still connected, overheating, drifting excessively, etc. This is one place where you might use that uncommitted op-amp.

And here is a variation on that circuit above that sends the VA+ and VA- signals into the ADC that is part of all data acquisition systems. If that system has an input mux in place, this circuit implementation is quite straightforward. This makes it easy to track and log these variations.

Intersil has released a nice Eval board to make it easy to experiment with the devices. Since one common use is based on a strain gauge bridge circuit, it provides a strain gauge device in its own board to make it easy to use. And the Eval board is not exactly a board — it's a “DAQ on a Stick,” or a data acquisition board on what looks like a jump drive.

The possible uses are pretty easy to see. For industrial applications, monitoring a strain gauge is commonplace. A similar application with a strain gauge is found in a weight scale. There are other industrial applications such as flow sensing and temperature or pressure monitoring. Medical applications include monitoring blood pressure, respiration, ECG, and glucose levels. For life support/self-contained breathing apparatus (for miners and firefighters), various gas detecting sensors are used. These IAs are just right for the interface between sensor and ADC. And since output current capability for op-amp No. 3 is 40mA, they won't have any trouble driving almost any ADC you put in its path.

The IAs have a two-bit port that allows you to change the gain. As noted above, different versions have different gain settings available, so check the data sheet for the details to see which one you need. Also, the devices have built-in RFI filtering to keep EMI under control.

Have you used IAs before? What uses do you have for parts like these?

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— Brad Albing, Editor-in-Chief, Planet Analog and Integration Nation Circle me on Google+

3 comments on “New Instrumentation Amplifier Makes Sensing Easy

  1. RedDerek
    November 6, 2013

    Brad, that is a nifty IA. I like the extra opamp application you described to watch for the VA+, VA- input changes. My only issue is that the supply is low for use in old electronics redesign. Otherwise, very useful.

  2. Brad_Albing
    November 17, 2013

    @RedDerek – yep, it would have been nice if at least the front end could have been powered by +/-15V; but even without that feature, it looks pretty useful.

  3. SunitaT
    November 30, 2013

    Instrumentation amps (in-amps) are exactness gain blocks which has a differential contribution and a production that might be differential, or single-terminated with esteem to an allusion terminal.

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