29 Bridges and Amplification

Two voltage dividers can be combined as a Wheatstone bridge to highlight very small changes in resistance.

Wheatstone Bridges

Two voltage dividers can be combined to highlight small changes in resistance in the form of a Wheatstone bridge. (video 7:14)

Although a Wheatstone bridge can be used to capture small changes in any type of resistor, we will be using explicitly in strain gauge circuits, to measure forces, stresses and strains. (Watch the video 9:41 or review the PDF)

Amplification

Amplification is a technique you will use repeatedly to increase the magnitude of small signals so that they are large enough to read with an analog to digital converter (ADC). Watching these videos will help you understand the ideas and be ready to make decisions in the labs. You will be coming back to this material, as we will use an INA125 Instrumentation Amplifier in labs on Load Cells, Thermocouples, and Pressure Transducers.

Small positive and negative signals in the mV range from load cells, thermocouples etc. need to be amplified and have a voltage offset applied to bring them into the 0-5 volt range of the Arduino ADC. (video 5:21)

The gain is the multiplier for converting input to ouput. For example, if the gain was 500, a 1 mV difference on the input would be amplified to a 500 mV difference on the output. In the common ground system the output would simply be 500 mV. If a pseudoground configuration is used with an offset voltage of e.g. 2.5 volts, that difference would be added, for an output voltage of 3000 mV or 3.000 volts. By examination we can determine the gain and offset need for a luggage scale load cell to be an input for an UNO. (video 2:56)

Finding exactly the right component can be difficult given the huge range of possible choices. Concentrate instead on finding a component that will do the job, fit into the system and have a reasonable cost while prototyping, and leave the optimization for later. Follow along the same path I did to choose the INA125 Instrumentation Amplifier to use in this course. (video 6:16)

It’s critical to insert the DIP (dual in-line package) chip carefully into the breadboard. Bent pins don’t make electrical contact and are hard to straighten. Once it’s in your breadboard, just LEAVE IT IN THE BREADBOARD! (video 0:36)

Wiring up the Amplifier is a skill you will need repeatedly. Once you get the basic wiring sorted, there are lots of reasonable solutions to select amplifier gains and ADC input ranges for thermocouple and other signals, but they need to match up. (video 4:07)

Using a lower value for the pseudoground allows for more range on the positive side of the scale, which makes sense as this load cell is almost never in compression. (video 2:43)

When the bridge has the same power supply as the ADC reference they will both go up and down proportionally to the power supply source. By choosing  wisely, we can get the variations to cancel out of the calibration process. (video 5:47)

Checking the Gain

We’ll find some errors when we check the gain on this amplifier circuit with a 56 ohm gain resistor. It should be 1075, but we only get 1060 when we measure. By measuring the gain explicitly we can improve our accuracy by about 1.5%. You will need to choose an appropriate gain for your amplifier to maximize the output signal while still staying below the saturation limit of the amplifier.The low resistance gives a high gain, but is affected by the variable resistance of the breadboard connections. (video 5:32)

Amplification Alternatives

If you are looking for a cheaper, but more complicated approach than using our favourite INA125 Instrumentation Amplifier, you could try the approaches below. You might want to get help from a friend in electrical engineering.

This tutorial shows how to turn an NPN transistor into a low gain common emitter amplifier, perhaps as a preamplifier for an audio signal. This is inexpensive and leads in an electrical engineering direction.

This application note from Analog Devices shows how to use an op amp as a preamplifier for a MEMS microphone. Careful balancing will be required to control DC offset.