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2.6.3.5 - Amplifying Small Signal Voltages

This section has general information about external signal amplification. The U6 has an outstanding amplifier built-in. Combined with the high resolution capability of the U6, an external amplifier is seldom needed, and in many cases will actually degrade noise and accuracy performance.

For a do-it-yourself solution, the following figure shows an operational amplifier (op-amp) configured as non-inverting:

Figure 2.6.3.5-1. Non-Inverting Op-Amp Configuration

The gain of this configuration is:

Vout = Vin * (1 + (R2/R1))

100 kΩ is a typical value for R2. Note that if R2=0 (short-circuit) and R1=inf (not installed), a simple buffer with a gain equal to 1 is the result.

There are numerous criteria used to choose an op-amp from the thousands that are available. One of the main criteria is that the op-amp can handle the input and output signal range. Often, a single-supply rail-to-rail input and output (RIRO) is used as it can be powered from Vs and GND and pass signals within the range 0-Vs. The OPA344 from Texas Instruments (ti.com) is good for many 5 volt applications. The max supply rating for the OPA344 is 5.5 volts, so for applications using Vm+/Vm- (±12 volts), the LT1490A from Linear Technologies (linear.com) might be a good option.

The op-amp is used to amplify (and buffer) a signal that is referred to the same ground as the LabJack (single-ended). If instead the signal is differential (i.e. there is a positive and negative signal both of which are different than ground), an instrumentation amplifier (in-amp) should be used. An in-amp converts a differential signal to single-ended, and generally has a simple method to set gain.

4 comments

Submitted by Anonymous (not verified) on Mon, 09/24/2012 - 12:36

Does the U6 require the gain of IN-AMP before the ADC be the same for all AINx?  In other words, if I have multiple inputs with different voltage range, can I set the IN-AMP gain different for different input?  Thanks.

Submitted by LabJack Support (not verified) on Mon, 09/24/2012 - 13:25

You can specify different ranges (i.e. gains) for different channels.  You can see in Section 4.3.3 that PUT_AIN_RANGE is an iotype, so it applies to a specific channel, and in the pseudocode you can see there is a line to set the range of AIN1 to +/-10V and another line to set the range of AIN2 to +/-1V.

Submitted by Anonymous (not verified) on Mon, 09/24/2012 - 15:08

Thanks for your reply.  That's very interesting.  Does that mean the IN-AMP is constantly switching between gain of 1 and 10 on the fly when I have two channels that are set to gain of 1 and 10? Will that cause any possible issues or problems? 

One thing I noticed in section 3.1 the speed is directly related to the gain settings. Gain of 1 gives you the fastest speed.  Is there anything else I should be aware of?

Thanks.

 

Submitted by LabJack Support (not verified) on Mon, 09/24/2012 - 16:26

Look at the block diagram in Figure 2-2 in Section 2.0.  After acquiring a reading from 1 analog input channel, to acquire a reading from a different channel the multiplexers change and the gain of the internal in-amp can change.  When these things change, the system must settle to the new conditions, so some settling time must be allowed for.  The U6 automatically allows for settling time, but high-impedance sources (>1kohm) might not settle properly.

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