Episode 19: ee-quipment's ee701 Differential Preamp

November 1, 2017

Video Transcription

Hi, this is Jack Ganssle, and welcome to "The Embedded Muse" video blog, which is a companion to my free "Embedded Muse" e-newsletter. Have you ever had a situation where you need to measure very low-level signals, say, in the microvolt range or maybe signals superimposed on a lot of common mode voltages? A new unit from ee-quipment is exactly designed for that application. This is a differential preamp for your oscilloscope.

Look at the unit a little more closely. You can see it has two differential inputs, one output that goes to your oscilloscope, a wall wart for a power supply, and various adjustments. This control just the gain in steps of 1, 10, 100, or 1,000. This is a cutoff frequency, which is a nice feature, and it can be AC or DC coupled.

One of the nice features about this is that the label describes exactly what is going inside the unit.

Most scopes have no more than one millivolt per division sensitivity. Many have far less than that. But another very important use of this particular device is for measuring signals with a large common mode component. In this case, I'm driving the two inputs from my signal, its arbitrary waveform generator, which has two outputs, two independent channels. But in this case, I've set them both to be a sine wave, and they're both linked together so that they're completely in phase. The only difference is that on one channel, I'm putting out 4.01 volts and the other channel exactly 4 volts. So there's only a 10 millivolt difference between the two channels. I've got the system here set to a gain of 1,000. And here, on the scope, you can see that the signal is basically enormous. That's two volts per division. It's a tremendous amount of gain. That is the 10 millivolt difference in voltage between the 2 sine waves.

This is a classic case where there's a tremendous amount of common mode voltage between the two signals. They both are riding basically on top of four volts, and we're looking for something very small that's the difference between them.

But there's more. Suppose you're measuring a signal which has a lot of noise on top of it. This differential preamp is designed to filter out that noise for you. I've increased the sine wave frequency to 100 kilohertz, and with the EE701 set to a one megahertz pass ban, you see that the signal gets through just fine. Same thing at 100 kilohertz. So I switch to one kilohertz, it's gone. It filtered out all that noise. So if you had any noise, whether it's common mode or not, this preamp is really good at rejecting it.

This product is hardly unique. Tektronix and others sell differential preamps for their oscilloscopes. Tektronix, for example, has their entry-level ADA400A has very similar specs to the ee-quipment device, but that costs $2,000 compared to under $400 for this unit.

Thanks for watching, and be sure to go to ganssle.com for over 1,000 articles about building embedded systems.