[time-nuts] Slightly OT: inexpensive USB analog-digitalconverter?

Didier Juges didier at cox.net
Fri Jan 18 12:35:30 UTC 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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