EM Electronics A23 (or similar model)

[RandallMcRee]

Hi folks,

I know that some of you have experience using these nice low-noise amplifier modules from EM electronics.

http://www.emelectronics.co.uk/a23.html

I would like to make a low-noise measurement between two voltage references. Is the A23 galvanically isolated like a DMM? If so, I guess I can simply hook the grounds of the voltage references together and feed output of one Vref to v+ of A23 and the other to ground of the A23.

If not, I would need an instrumentation amplifier to remove the common mode voltage, correct?

Edit--One more question if I may: the 1 ohm feedback resistor that is internal to the unit is specified at 15ppm. That seems to be non-optimal for precision measurements over time...has anyone found this to be a limitation? or is it small potatoes compared to other factors I have not mentioned??

Thanks for any insight.
Randall

[Echo88]

I dont know if the A23 is galvanically isolated and i think if your well matched (A23 2mV input max) references are galvanically isolated itself to allow difference measurements, then it shouldnt matter.
Apart from the A23-solution may i suggest the Keithley 181 Nanovoltmeter, which comes for about 250€ used on ebay very often. The A23 has 1.3nVpp, the K181 ~15nVpp AFAIR and i dont think that you need the ultralow-noise-voltage of the A23 when you compare voltage references, since even the chinese 2DW-zeners have ~300nVpp and drift due to age/temp.
I dont really see the use-case for the nV-amplifier; is your goal really only noise measurement or do you want to see the tiny reference-drift effects during short scales?

Thinking further:
Wouldnt in this case a Low Noise Amplifier 0.1-10Hz be the better solution, like the ones shown in this thread: https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/ ? They are made to determine the low frequency voltage noise of references and are capable of having a amplifier-noise-floor of ~150nVpp RTI (if we take Andreas-Design as an example) and therefore able to measure even the small 2DW-zener-noise.

Sorry if that wont answer your questions regarding the A23, im just curious.

[chuckb]

I have experience with the A10, A22, and A23 amplifier modules from EM Electronics. They do not have any isolation between the input and output or the power supply.
I power the modules from either two batteries or a small power supply. The current draw is less than 5 ma. In my setups the amplifier is battery power and directly feeds a DVM because it’s input is floating also. The temp co of the 1 ohm resistor is not an issue except in the most extreme situations. In the extreme case of a 3mV difference between voltage references and the amplifier temperature changing 4 deg C the resistor temp co would cause an apparent 0.2uV change at the input. This will still be below the noise level of two Zener references.

A simpler solution would be a K181 Nanovoltmeter like Echo88 suggested or a simple LT1167 Instrumentation amplifier setup with a gain of 1000 supplying a normal DVM. The LT1167 costs less than $10 USD.

[RandallMcRee]

Thank you both!

I am going to be experimenting with a USB DAQ from Janas Card

http://www.janascard.cz/aj_Vyrobky.html

These do have galvanic isolation, which is very helpful, I find.