Charge pumps, PWM, and FETs

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I have a resistance I'd like to measure, and my main choices seem to be:

A noninverting op-amp with a fixed reference as the input and the sensor in series with the feedback resistor and ground, Vout = Vref*(1+Rf/Rs). The problem here is the reference voltage eats about a bit of ADC resolution. I can live with it, but it's not ideal. Cutting the reference down with a trimmed voltage divider is an option, too.

The circuit recommended by the sensor manufacturer is a inverting op amp, with a negative voltage through the sensor into the input, Vout = -Vin*Rf/Rs. The trick here is the negative voltage.

There are a ton of threads on getting a negative voltage to drive an op amp, but I'm not doing that here. This is a single supply mcp600x, with a negative input.

I'm short on space, but I can have a charge pump running off the AVR's PWM, using two diodes and two caps. This isn't regulated, but do I care, if its only juicing the inputs to some op amps? The datasheet says 19 pA typical input bias current, but no max. I'd be using eight channels. The sensor resistance would vary between several kohms and seveal Mohms.

I can choose an op amp with more detail in the datasheet, too. They aren't ordered yet :)

Some of the charge pump circuits I'm stumbled across (the more authoritative ones, generally) run the PWM through a FET and that charges the caps. When would this be useful, other than to switch a higher voltage, which I don't need to do, and what's a good FET for the job?

I did find a web page with a neat H-bridge circuit and an AVR that used RFD3055s: http://www.dharmanitech.com/2008.... This example looked just right, since it's breadboard friendly and doesn't use a driver.

TIA!
Chris Fuller

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Wait... the clouds are clearing in my crystal ball... I see a Negative Temperature Coefficient thermistor.... You are making a thermometer! And you are in luck. I myself have mastered this device using an AVR. My NTC resistance varies about 3k at room temp down to a couple hundred ohms when hot. I have a 1k from 5v to the ntc, which is grounded. I have an AVR a/d input on the NTC. Read the volts. Calc the current. Calc the R. Voila! Bob's Yer Uncle. Oh. Now you want the Steinhart equation to solve for temperature? Just ask.

Imagecraft compiler user

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Quote:
This isn't regulated, but do I care, if its only juicing the inputs to some op amps?

If your ultimate reading depends on -Vin, as shown in the equation, then I would have thought you would care. As far as FETs go, I've used simple diode pumps(just diodes and caps) in the past to provide a negative voltage for RS232, and that was without a PWM output, the program(on a P*C) toggled an output pretty much continually. In the end I suppose the current needed depends on the sensor resistance, the op-amp input inpedance and the feedback resistor.

Four legs good, two legs bad, three legs stable.

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Uncle Bob, you are a better man than I am Gungar Din. I thought that he was wanting to use a H-bridge to charge some caps to generate a negative voltage. I wonder where I got that idea? :lol:

Ross McKenzie ValuSoft Melbourne Australia

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bobgardner wrote:
Wait... the clouds are clearing in my crystal ball... I see a Negative Temperature Coefficient thermistor.... You are making a thermometer! And you are in luck. I myself have mastered this device using an AVR. My NTC resistance varies about 3k at room temp down to a couple hundred ohms when hot. I have a 1k from 5v to the ntc, which is grounded. I have an AVR a/d input on the NTC. Read the volts. Calc the current. Calc the R. Voila! Bob's Yer Uncle. Oh. Now you want the Steinhart equation to solve for temperature? Just ask.

Bzzzzt! Sorry, wrong answer, but here, have some fine consolation prizes..

It's a Flexiforce pressure sensor. I've fiddled a bit with thermistors and have that Steinhart equation down left, right, frontwards and back, thanks!

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Shucks. My crystal ball was all cloudy. Anyway, measuring the V across and the I thru a known R feeding your transducer is a pretty foolproof way of determining the unknown R. I made an xy joystick out of 2 pairs of those force transducers, with two in series in x, another 2 in y. The joystick would squeeze either the top one or the bottom one, making the volts in the center go up or down. Clever huh?

Imagecraft compiler user

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Even if you aren't using the negative supply voltage to as part of your equation, you still need to have it relatively clean as some noise on it will leak through into the output of the device. Your op-amp's datasheet should have a graph of -PSRR.

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I understood the OP to say that he was using a single rail op-amp and the negative voltage was NOT for the supply.

Four legs good, two legs bad, three legs stable.

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I am presuming that the noise can be mitigated by a suitable choice of capacitor. I'm thinking I may not need regulation because the current drawn will be very low.

The nice thing about (non)-inverting circuit is the applied force is proportional to capacitance (1/R), so the measured force is a linear function of Vout.

I'm not sure what you're proposing, Bob. I can't use a meter in the final application, and if I use one in development, I should just set it to resistance directly. Anyway, there'd be a large uncertainty with the tiny current going into an ADC input.

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My eye caught the bit about wanting to read a resistance, and I recently worked on how to read an NTC thermistor... I ran 5v thru a 1k resistor to the ntc to gnd. I read the volts on the ntc with the a/d. I can now calc the i thru and the v across the ntc. The you mentioned that you were using the varuable resistance force xducers, and I thought I'd describe my use of them in a joystick, which might be useful to someone... just hook em in series from 5v. Middle is 2.5. When either one is squeezed, the volts moves that way.

Imagecraft compiler user

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I realized all the voltage drops were in parallel, so won't accumulate, then did the Ohm's Law thing with my least significant bit, got a ceiling for the max input bias current, and found an inexpensive (2x the mp6002, but that was just 36 cents) dual channel op-amp, with a max that fits: TI TLC272CP.

Sorry to bug you guys, but maybe this thread will help somebody else. Cheers!

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One method that I haven't seen mentioned would be to use an LM317 voltage regulator and a resistor wired up as a constant current source. Connect the sensor from the output of the current source to ground. The voltage across the sensor is directly proportional to the resistance of the sensor.
The power supply required is 3-4 volts higher than the largest voltage drop across the sensor, and the regulation wouldn't be particularly critical. Just choose the resistor to give an appropriate current through the sensor. Calibration can be done either by trimming the current setting resistor or simply inserting known calibration resistors in place of the sensor and recording the readings in EEPROM.