Converting +/-50V source to 0-5V via diff amp?

Yea, instant poof. :-) In my first paragraph (in OP) I had discounted using the voltage divider and was leaning towards a diff amp but hadn't used them before...lots of questions resulted and the adventure began.

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Okay- well it's really not viewed as a divider per se, it is two parts. The 2 x 120K form a common mode summer, meaning the voltage at their junction is (Va+Vb)/2=Vcm- their common mode. Then from the perspective of the 15K node, this looks like a Thevenin source of 120K/2=60k output impedance with open circuit voltage Vcm. The 15K then loads this down to Vcm*15/(15+60)=Vcm/5=(Va+Vb)/10. So it doesn't make any difference which of Va or Vb is connected to GND or the source you are monitoring, the IN node will read |Voltage|/10. This assumes your circuit loading of the IN node is small compared to 10K, an easy thing to do. There will be two main types of common interference into your measurement. The conducted type due to currents circulated between your measurement source common and the PIC GND will be negligible because that current divides in the ratio of approximately 120K to probably milli-ohms on frame or whatever, so this will be down 160dB or something ridiculous. The second major interference will be electric field coupling injection currents into your connecting wiring. Generally the effective coupling capacitance is on the the order of 10's pF, meaning only high frequency fields will have any kind of measurable effect. Shunt the 15K with 0.1u to kill these by 80dB. Simply twisting the Va/Vb wiring buys immunity from magnetic induction coupling into the circuit. You would have to use that in any case because that type of interference is differential.

Thanks Fred. Good tips on reducing the noise too.

But doesn't the circuit require that the negative terminal for the SOURCE and the ground for the PIC's power supply be connected? Then I can swap the SOURCE's +++ and --- leads with the Va/Vb connection points and the output is always +5V (with respect to PIC ground). Otherwise, if the SOURCE neg term. and the PIC ground are not connected, no current flows thru the 15K resistor and I never get my

5V.

The SOURCE pos. and neg. connections to the PIC circuit might be made either way so the two grounds might not be connected together. And if I connect PIC ground to Va or Vb, IN is +5V or -5V (with respect to PIC ground), depending on how the SOURCE is connected.

Am I missing something? I breadboarded the circuit but still had the same results. Thanks!

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Just a FYI.... I tested the schottky diodes I have (11DQ05) over the temperature range they might be subjected to during use (25-70 degrees C) and the voltage dropped over 0.1 volts as the temperature rose.

While a fixed voltage drop is no problem (as long as it's less than

0.4 or so) and can be compensated for in software, a varying voltage is a problem and prevents me from using an inline diode.

A fuse and a beefy reversed schottky across the input connections is looking better and better.

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[snip]

Try this? R4 R5 +5v---------+---/\\/\\--+--/\\/\\----+-->Vout | | | \\ | +5v | R3/ | __ | | \\ +--|- \\| | R1= 10*R. R1 | |Op >---+ Vin---/\\/\\---+------------|+_/| R2= 10*R/9. |\\ | \\ \\_Voltage, V1. | R3= R. R2/ | R4= R. \\ | R5= R. | | 0v----------+----------------+------0v

When Vin= +50, V1=5. When Vin= -50, V1=0.

The opamp has to be rail-rail input and output.

--
Tony Williams.

Thanks Tony, Been crazy busy this week but will try it this weekend. If I can swap the leads from a 50V source on Vin and 0V and have a single-ended Vout, I'll be a very happy man. :-)

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In article , John wrote: R4 R5

Just to clarify.

Vout= Vin/10 for Vin= 0 to +50V.

Vout= 0V for Vin= 0 to -50V, but no damage to the opamp.

There could/will be damage to the opamp if Vin is connected when the opamp is not powered. Protected by the addition of two Schottky diodes, across R2 and R3, both pointing upwards. Protection also helped by a large value for R1, say 100k or greater.

--
Tony Williams.

Or do it right....

I think the U3 and U1 functions can be combined into a single stage, but I don't have time to play with it right now.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

In message , dated Fri, 8 Sep 2006, Jim Thompson writes

What are you playing with instead? Put it DOWN, boy!(;=)

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
There are benefits from being irrational - just ask the square root of 2.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Smirk ;-) Look around Big Boy, and see if you missed anybody ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

This is what I mean about your SED circuits. You /can/ combine it all into one stage, did it several days ago, but with those small input currents, 10uA on the low end of Vin=1V, you need low leakage diodes, maybe even JFETs diode connected. I gave up on this OP, he says one thing but then goes on to display zeroid ability. TW's circuit is good but I think it's too risky for a zero ohm connection to an input lead.

Show me. Message ID ?:-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Shucks, and I was really hoping to learn something here. :-) You don't even want to tell me why I didn't get the results I thought I was going to with your circuit? Was I supposed to know something about your circuit that you didn't mention?

Zeroid ability would mean I couln't even breadboard your circuit to try it out. Or worse yet, wouldn't even want to. But I did want to and did take the time to test your circuit.

Politely asking questions of those who know more than I and being willing to take the time to test out recommended circuits, that's a lot more than zeroid ability. I never deserved a bit of the sarcasm and brushing off I've gotten from you and was sincerely hoping you could help me.

I'm more than willing to do the leg work here, just needed some pointers. I'm damn sure you once knew less than you do now and that you have asked many, many questions about EE stuff in your lifetime. I'm doing nothing more than you have already done and would hope that you could give me the same basic respect that you deserved when you asked questions from those who knew more.

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Thank you Tony. I've been thinking about the circuits I've been playing with, and yours, and realized that Vout would stay at 0V. This is definitely no problem. :-)

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What exactly is your 1-50V voltage source that can be accidentally wired in reverse?

Is it truly floating, (as my circuit requires), or does it have a Gnd/0v connection somewhere, (as JT's or Fred's differential circuits can cope with)?

--
Tony Williams.

My version DOES handle a floating OR grounded source.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I sent an ASCII schematic to snipped-for-privacy@nospam.net but it kept bouncing:-)

You do entirely too much talking and far too little actual work to be worthy of any investment in time from others.