Discussion in 'Electronic Design' started by RealInfo, Apr 8, 2013.

2. ### Bill SlomanGuest

Uses witches to connect any one (or more) of a range of resistors
between the input terminals.
It could be latching reed switches, or it might be semi-conductor
switches - most likely big MOS-FETs.

3. ### Bill SlomanGuest

What's spooky about switches? It's an obvious typo - though not
obvious enough for me to have caught it.

I got it right twice in the line you snipped

"It could be latching reed switches, or it might be semi-conductor
switches - most likely big MOS-FETs. "

4. ### Phil AllisonGuest

"Bill Sloman"
Uses witches to connect any one (or more) of a range of resistors
between the input terminals.

** Did you bother to read the specs ?

At all ?
It could be latching reed switches, or it might be semi-conductor
switches - most likely big MOS-FETs.

** The max input power is 1W.

I suspect one or two, digitally controlled MOSFETs are involved.

The specs leave far too much to the imagination.

Don't even say what the voltage ratings are.

.... Phil

5. ### Spehro PefhanyGuest

Imagine a 4-quadrant MDAC with the (buffered) voltage on the terminals
as the reference. Say the DAC controls a bipolar current source.

Load the DAC with a code representing G= 1/R(simulated) and you're
done (conceptually, anyway), since I = V*G = V/R(simulated).

Range switching probably uses small relays.

There will be an interesting array of possible error sources etc. in
this sort of thing, but most instrumentation uses 10uA-10mA currents
and reasonable voltages.

6. ### Bill SlomanGuest

Which is why I fancy latching relays or MOS-FET switches and an array
of real resistors. There aren't any significant DC off-sets in that -
give or take some thermocouple voltages, and with latching relays or
MOS-FETs you aren't dissipating enough power to generate much in the
way of temperature differences.

There's nothing particularly horrible in that kind of solution. Your
16-bit MDAC solution probably offers finer resolution, but at a
considerable cost in circuit complexity and extra error sources.

7. ### Bill SlomanGuest

The unit is powered by four AAA cells - despite Phil Alison's
scepticism I did read the specs. That's compatible with switching
using latching relays, but I don't think you'd get much battery life
if they were being used to keep regular relays closed.

8. ### John DevereuxGuest

"real resistors" is also more their speciality - they manufacture
precision resistance standards.

9. ### Syd RumpoGuest

Yes, Bill. It was what's known as a 'joke'. Not a very good one,
admittedly, but also not a slight on your integrity, ability or virility.

Cheers

10. ### Bill SlomanGuest

That makes a change. Some of our regulars don't seem to feel that a
post is complete until they have managed to impugn all three.

11. ### John DevereuxGuest

Dunno, maybe. But it is strange there is no specific voltage limit as
far as I can see. There is a simple 1W dissipation limit, like you might
get if you used 1W resistors. And there are "30 calibration resistance
values" to set during calibration. Which is about what you need for a
binary stepped resistance chain isn't it? 0.1ohm to 24M, ~30 bits.

But there are other explanations for all the above.