[time-nuts] Slightly OT: inexpensive USB analog-digitalconverter?
Didier Juges
didier at cox.net
Fri Jan 18 07:35:30 EST 2008
The LTC6102 is even better (did not notice it before) with 10uV max offset.
That lets you use a very small series resistor (in the milli-ohm range) and
still have milliamp resolution and several amps of full scale dynamic range.
I am in the process of doing exactly what you are talking about (power
supply remote monitoring and control). I currently have a Maxim current
sense chip in my power supply (MAX471, no longer manufactured) which I will
replace with probably the LT6102, the processor is a TI MSC1210 which, for
all its quirks, has a 24 bit A/D converter built in. The voltage reference
is an AD580, not the best but sufficient and I have several of those.
I have used that chip before to plot the current draw versus temperature of
an HP 10811's oven. The temperature was measured using a 10k NTC thermistor
and the A/D converter of the same TI chip:
http://www.ko4bb.com/Test_Equipment/HP10811-Current.png
Here is the link to the Linear Tech page on high side current sense (again):
http://www.linear.com/ad/current_sense.jsp
Didier KO4BB
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Didier Juges
> Sent: Friday, January 18, 2008 5:26 AM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Slightly OT: inexpensive USB
> analog-digitalconverter?
>
> 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)
>
> Didier KO4BB
>
>
> > -----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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