Using the SA44B at very low frequencies.

I need to use my SA44B at as low a frequency as possible (10Hz or so) whilst at the same time blocking DC. I could just put a 49.9 Ohm resistor, 2.2uF capacitor (what type-Presumably NOT electrolytic) in series and then a resistor directly between the SA44B input and ground (what value?). This was suggested elsewhere on this forum. However, I’d like to minimize measurement errors and I’m wondering if this is the best way? Any suggestions? Thanks.

If your lowest frequency is 10 Hz, and your highest frequency is 100 kHz, and electrolytic will probably work fine. If you need to go up to a MHz or beyond, a polymer cap will work. For 10 Hz, you would be looking at something like 200 uF – 470 uF.
A 1 kohm resistor to ground after the cap will help discharge the cap AFTER powering on your device and BEFORE connecting the SA44B. A 50 ohm series resistor somewhere is desirable if you have a low impedance output.

Hopefully this helps.

Thanks for your reply.
Could a 1:1 isolation transformer or an opto-isolator be used? It’s mainly the 10Hz to 100KHz region that I need to cover as the BLK-89+ DC Block will be Ok from then on. What do you use yourself for coupling such signals? Thanks.

Last time I needed to AC couple down to 10 Hz, I used a 220 uF aluminum polymer cap for DC blocking with a 1k to ground after, using two SMA connectors and a soldering iron. It wasn’t pretty, but it worked.

[Justin Crooks]

I guess I should add that an isolation transformer could, in theory, work, but one that performs well down to 10 Hz might be very hard to find. As for an opto-isolator, my primary concerns would be linearity and noise, and you would likely still have to DC block.

• This reply was modified 6 years ago by Justin Crooks.

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