EM Electronics A23 (or similar model) - Page 1
Forum section: electronics / general

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 25, 2018, 01:24:52 am
content: 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.htmlI 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

author: Echo88
author_rank: Frequent Contributor
timestamp: October 25, 2018, 02:09:20 am
content: 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.

author: chuckb
author_rank: Frequent Contributor
timestamp: October 25, 2018, 01:58:20 pm
content: 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.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: November 02, 2018, 04:45:44 pm
content: Thank you both!I am going to be experimenting with a USB DAQ from Janas Cardhttp://www.janascard.cz/aj_Vyrobky.htmlThese do have galvanic isolation, which is very helpful, I find.