# Strain Gage Amplifier

Discussion in 'General Electronics Chat' started by FlyingSwissman, Apr 6, 2009.

1. ### FlyingSwissman Thread Starter New Member

Apr 6, 2009
6
0
Hi everyone,

I'm new here and I'm hoping you will be able to answer a question of mine . I have a strain gage connected to a Wheatstone bridge the excitation and output voltage of the bridge is connected to an amplifier. My problem: I am not sure how to determine the amplifier gain The datasheet of the ampliefer can be found here: http://www.blh.de/pdfamp/ba660_e.pdf

I would really appreciate if someone could tell me how I can determine the gain.

FlyingSwissman

2. ### gotumal Active Member

Mar 24, 2008
99
0
I think, the gain has not been specified directly. Instead the sensitivity of the strain guage bridge to be connected is given as 1, 2 or 3.5 mV/V. i.e. change inmV per volts of excitation. You could calculate based on the excitation given; for the output voltage of ±5V.

3. ### FlyingSwissman Thread Starter New Member

Apr 6, 2009
6
0

I know my amp has a sensitivity of 2 [mV/V]. So if for example:

- Power Supply = 6.5 [V]

- Analog Output = 2.5 [V]

-> Bridge Input = ? [mV]

I simply want to be able to derive the bridge Input from the Analog Output of the Amp.

-Excitation given: 5 [V] (what Power supply Voltage is required for this excitation to present for certain?)

-> ± 10 [mV] = Bridge Input (due to Sensitivity)

- ± 5[V] = Analog Output of Amp

-> 10 [mV]*G=5000 [mV]

-> G=500 (?)

I think I might be missing a fundamental point here...

Thanks again,

FlyingSwissman

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The strain gauge bridge output should be linear. Put a known load on the bridge and observe the output of the amp. Change the load by a known amount, and observe that output. Those measurements should allow you to derive the output of the bridge.

5. ### profray New Member

Apr 6, 2009
2
0
I agree with the last post. Calibrate it all together with the final product. Sensitivities and gain values dont mean much until you put it to the test. The output will be directly proportional to the power supply as well. You can sometimes use the same power source as a reference supply to your a/d converter board so that you are still working with ratios of supply/output instead of absolute voltages.

6. ### FlyingSwissman Thread Starter New Member

Apr 6, 2009
6
0
Ok, thanks. I measured the Analogue Output of the amp and compared it to the Bridge Voltage expecting a linear relation (Power supply @ 6.5 [V]). Here is what I got from three measurements:

1.
----------------------------------------------------------
Bridge Input (to amp) = 4.2 [mV]
Analog Output (of amp) = 2491[mV]

-> Gain Factor=593

2.
-----------------------------------------------------------
Bridge Input (to amp) = 8.4 [mV]
Analog Output (of amp) = 3619[mV]

-> Gain Factor=431

3.
-----------------------------------------------------------
Bridge Input (to amp) = 2.2 [mV]
Analogue Output (of amp) = 1493 [mV]

-> Gain Factor=678

The gain Factor does not seem to be constant, even though the power supply was kept constant at 6.5 [V] meaning the excitation is also constant.

I have looked at the Vishay website and I am using one of their strain gages.