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Let’s Talk About the Low Noise Measurement Probe


Ever tried to measure low-level signals in the presence of noise? Or wanted to capture a measurement isolated from ground, like that of an ungrounded hand-held digital multi-meter, but also needed to capture finer details of the signal, like those captured with an oscilloscope? Are you a power supply designer wanting to measure power supply ripple on a switched-mode-power-supply? If you’re in any one of these camps, please read on. This article will show you one way to accomplish these goals using an instrumentation amplifier, a development/evaluation board, a small metal box, a BNC cable, two 9V batteries and a ferrite. First, some background information on instrumentation amplifiers is provided.

Background on Instrumentation Amplifiers

An instrumentation amplifier is used to amplify very low-level signals, rejecting noise and interference signals. Examples of the signals that can be measured are heartbeats, blood pressure, temperature, earthquakes, and so on.

Inputs to instrumentation amplifiers will have very low signal energy therefore, the instrumentation amplifier should have high gain and should be accurate. The gain should be easily adjustable using a single control. It must have high input impedance and low output Impedance to prevent loading.

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The instrumentation amplifier should have high common-mode-rejection-ratio (CMRR) since the transducer output will usually contain common mode signals such as noise when transmitted over long wires. It must also have a high slew rate to handle sharp rise time events and to provide a maximum undistorted output voltage swing.

Low Noise Measurement Probe Details

Figure 1 shows a low noise probe that you can easily build yourself.

Figure 1: A Low Noise Measurement Probe

This probe is essentially a 10 MHz low noise true differential instrumentation amplifier. It is battery powered so it can float with respect to the signal being measured. This feature allows you to take differential readings, even if the measured signal is a voltage with respect to its own local ground. It’s able to measure signals without connecting the signal ground to scope earth ground by the true differential input and by virtue of being powered off batteries.

It’s got a high common mode (CM) range and good CMRR that helps with isolation of the measured signal from the scope ground. The CM range is +/- 36 V while being powered off of two +/- 9V batteries.

The instrumentation amplifier used in this probe is from Analog Devices Inc., model AD8421. It is installed on a development board specially designed to accommodate the AD8421. The development board is model EVAL-INAMP-82RZ and is also designed by Analog Devices.

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Some of the important specifications of the AD8421 instrumentation amplifier are:

  • Low power
  • 3 mA maximum supply current
  • Low noise
  • 2 nV/√Hz maximum input voltage noise at 1 kHz
  • 200 fA/√Hz current noise at 1 kHz
  • Excellent ac specifications
  • 10 MHz bandwidth (G = 1)
  • 2 MHz bandwidth (G = 100)
  • 6 µs settling time to 0.001% (G = 10)
  • 80 dB CMRR to 20 kHz (G = 1)
  • 35 V/µs slew rate

Installed on the development board is a jumper that allows selection of unity gain or 10x gain. The development board is provided with a user’s guide, UG-261, which will come in handy should you attempt to put together your own low noise measurement probe.

​The development board and instrumentation amplifier are placed into a metal box that has following dimensions: 4.25″L X 2.64″W. If you would like to purchase the exact box shown to build your own probe, the manufacturer is Pomona Electronics and it has part number 2902.

One of the cables shown on the right in Figure 2 connects to the instrumentation amplifier + and – inputs and has a third ground shield wire that’s also attached to the metal box. It’s probably a good idea to keep the input cable length as short as practically possible to lessen their ability to pick up external noise. Notice the ferrite common mode choke installed around the cable to suppress any common mode noise that may get onto it.

Figure 2: Input Cable

The output cable shown on the left tin Figure 2 connects to the oscilloscope input. It’s just a standard 50W BNC cable that can be found in most EMC laboratories.

The battery holder is made by Keystone Electronics and has part number 1295.

Figure 3 shows how all of the low noise measurement probe parts are connected internally.

Figure 3: Inside of Low Noise Probe (AD8421 installed on EVAL-INAMP-82RZ)

Parts List Summary

  • Analog Devices, AD8421 Instrumentation Amplifier
  • Analog Devices, EVAL-INAMP-82RZ Evaluation Board
  • Pomona Electronics Metal Box, 2902
  • One 50W BNC Cable
  • Keystone Electronics Battery Holder, 1295
  • Two 9V Batteries
  • TE Connectivity AMP Connectors
    • Conn Socket 22-26AWG Crimp Gold, 87666-2
    • Conn RCPT Housing 2pos Crimp, 925369-2
    • Conn RCPT Housing 3pos Crimp, 925369-3

References and Further Reading

  1. Electronics|Projects|Focus. What is an Instrumentation Amplifier? Circuit Diagram, Advantages, and Applications.
  2. Analog Devices Inc., AD8421.
  3. Analog Devices Inc., EVAL-INAMP.

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