[time-nuts] Slightly OT: inexpensive USB analog-digital converter?
didier at cox.net
Fri Jan 18 06:25:51 EST 2008
Be careful that if you do not isolate the A/D converter analog circuits from
the USB ground, you may create all sorts of nasty ground loops with the
computer. When dealing with power supplies, it's always best to be isolated.
I do not know enough about USB to know if it can be simply isolated with
opto couplers, but I know that RS-232 is very easy to isolate. So if your
data rate is not great, a serial solution would be easier to implement (even
if you go through a serial-USB adapter for convenience on the PC side)
Linear Technology, Analog Devices and Maxim have hot side current sense
chips that are accurate and cheap. They are specialized diff amps, not
regular IA. LT for instance has one that has less than 50uV offset (LT1787,
see Design Note 227)
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Hal Murray
> Sent: Friday, January 18, 2008 3:55 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Slightly OT: inexpensive USB
> analog-digital converter?
> > (My goal is to add digital voltage and current monitoring to some
> > older bench power supplies.)
> Measuring voltage is simple. If the input range is too big,
> you need a few resistors. If it's small, you lose a few bits
> from the top of the A/D. Add an op-amp if you need them.
> Measuring current gets tricky.
> There are several chips designed for measuring current on 48V
> busses. I think they are just an instrumentation amplifier
> with big enough resistors on the front end so that the common
> range goes up to 48V.
> Alternatively, if the supply is floating, you can put the
> sense resistor in the ground lead, but you still need a good
> amplifier to make the signal big enough for an ADC.
> These are my opinions, not necessarily my employer's. I hate spam.
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