# pseudo differential input of amplifier

The Signal Recovery Model 5184 is an ultra low noise preamplifier. The product description says

... and incorporates a special pseudodifferential input stage that can be floated to give the ground loop immunity normally associated with true differential inputs but without the associated noise penalty.

The question is

1. What is a pseudo differential input.
2. Why is there a noise penalty for real differential inputs.
• I don't doubt they have some sort of not quite differential input, but the rest sounds like Marketing Speak to me. I can't off the top of my head figure out why a fully differential input would have increased noise over some type of pseudo-differential scheme. Maybe if the true differential scheme requires more stages of amplification or more components but that's not in evidence. – John D Jul 29 '15 at 15:45

Typically pseudo-differential inputs have an asymmetrical input impedance- much lower for the 'low end' of the input. In this case, it only allows 300mV of common mode voltage. I didn't read the manual fully, but some instruments have 50 ohms relative to ground, which is high enough to prevent serious interference from ground loops.

Of course a fully differential input stage will have a higher noise, the textbook amount is $\sqrt{2}$ higher for two inputs subtracted with uncorrelated noise source in each.

This particular instrument has a JFET input with very low white noise (0.8nV/$\sqrt{\text{Hz}}$ @ 1KHz), so the ~40% improvement is worthwhile.

Note that they are targeting cryogenics, where the classic Johnson-Nyquist source noise will have much lower power for a given source resistance than at room temperature (noise power spectral density is proportional to the absolute temperature), so you would like to have an amplifier with a low noise temperature and high input impedance. That combination might not make sense at room temperature, at least without a transformer.

Edit: Without getting into the exact topologies of a low noise preamp- the pseudo-differential input low is essentially an internal ground- there is zero noise from it- by definition- because it is the reference point. The high input goes to the JFET gate, and a good JFET contributes noise of < 1nV$\sqrt{\text{Hz}}$ above the corner frequency. A differential front end would require two JFETs (such as a differential long-tailed pair) and each would contribute similar amounts of noise, so 1.4 times higher noise. I'm ignoring the exact topology and the necessary feedback path, but it should be obvious that the feedback path can be via a passive low-impedance divider (for example, to the JFET source) and thus will not be a contributor to the noise other than the noise from the resistors (which can be made relatively small- perhaps a few ohms- so its noise contribution is << 1nV$\sqrt{\text{Hz}}$. Ground-referencing the signal would take place after amplification, so the noise contribution from that is negligible.

For a possibly more familiar example using op-amps, compare these two:

simulate this circuit – Schematic created using CircuitLab

The input-referred noise of the single-ended gain of 100 amplifier will be approximately 1/sqrt(2) of the noise of the fully differential instrumentation amplifier with the same gain because the $e_n$ of the two amplifiers add in quadrature.

• Why would a pseudo-differential input not result in sqrt(2) increase in noise? Does it not subtract the signals from the two inputs? Does it not have noise on both inputs? Is the noise correlated for some reason? – The Photon Jul 29 '15 at 16:01
• I can see the lower noise of a single-ended input vs. differential, and if the single-ended approach also allows the inputs to float (pseudo-differential?) I can see that helping with breaking any ground loops. I guess the downside would maybe be that the CMRR might not be as high as a true differential input so common mode noise might not be rejected as well? – John D Jul 29 '15 at 16:01
• @The Photo You'd need to have two JFET inputs rather than a single JFET (or set of JFETs), so the noise would add (typically in quadrature). The other way, it's just a single amplifier with one low level input (and a reference level). Compare an op-amp amplifier with an instrumentation amplifier. – Spehro Pefhany Jul 29 '15 at 16:05
• @JohnD The CMRR should be as good (maybe even better) but the asymmetrical impedances may have an effect on what gets to the amplifier input terminals. – Spehro Pefhany Jul 29 '15 at 16:07
• I still don't understand what is meant by a "pseudo-differential input" and why it has lower noise. – The Photon Jul 29 '15 at 16:13