I need to make pressure measurement circuit. Output of this circuit must be a variable resistance that must be between 10 ohm and 180 ohm acoording to applied pressure. pressure sensor gives 0- 20 mV output and I amplify it by an instrumentation amplifier to 0 - 5V level and sample it with an microcontroller. After microcontroller stage I must convert this measured value to an resistance value for output because Display circuits reads pressure values as a variable resistance between
10 ohm to 180 ohm. Variable resistance is referenced to ground from one of its terminals. Maybe this design can be implemented without microcontroller.
How can I make variable resistance by the means of microcontroller or without using microcontroller
Simplest is often PWM duty cycling which integrates into resistance.
--
Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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If Don's pure PWM doesn't work you could use PWM and drive a FET gate with it. Make sure it's integrated well enough so the gate sees pure DC. However, since these have a large tolerance you need to drive via an opamp, use a FET array and hook a 2nd FET on the same die into the feedback. The 2nd FET forms a voltage divider with a resistor and then the uC can simply set a DC voltage (via the PWM) and this value translates into drain-source resistance.
You have the microcontroller, use a PWM output, switch between two stable known voltage values (Vdd/Vss may be good enough for you), LPF to get DC, feed that to a voltage-controlled current sink. Measure the voltage at the current source output pin using a spare ADC input. Periodically calculate and output the PWM duty cycle so that Iout = Vc/Rs, where Rs is the desired simulated sensor resistance and Vc is the voltage at the collector (or drain) of the current sink.
Best regards, Spehro Pefhany
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A 1K resistor with a CMOS 4016 or other switch in series with it looks like 1K at full duty cycle, 10K at 9:1 and 100K at 99:1 duty cycle.
So long as whatever is using the resistance can integrate the pwm. Usually a plain old capacitor will suffice.
--
Many thanks,
Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com
Please visit my GURU\'s LAIR web site at http://www.tinaja.com
Farnell list several interesting devices - Fairchild's opto-FETs (H11F1 and H11F3) which make very good isolated controllable resistors, though they won't get down to 180R, let alone 10R, and Vishay's IL300 optocouplr with two separate photodiodes, both driven by the same LED.
With the IL300 you use one photodiode for local feedback, while the other one sits on the other side of the isolation barrier. Cute.
He needs to go down to something like *10* ohms, sounds like an automotive gauge or something like that. I'm thinking those things respond to the RMS current, not the average current, since they use a heater coil and a bimetal.
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
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Embedded software/hardware/analog Info for designers: http://www.speff.com
These are nice but not so suitable for RF paths because of their large GS capacitance. A real MOSFET might be better in many cases if you can live within the limits for the gate breakdown voltage.
If I were emulating variable resistance I think I'd opt for a multiplying DAC, an OpAmp and a FET (or bipolar), so that the devices expecting a variable R actually saw the "real thing". I'm not sure some circuits would respond well to a PWM emulation unless designed for it in the first place.
...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 |
It\'s what you learn, after you know it all, that counts.
Thank you everybody for your advices. This circuit will be used in automative application. Circuit must work in wide temperature rance maybe -4 F and 176 F (-20 C and 80 C) so if I use FET type solution Linearity and resistance change can occur. if I use this type of solution I will need a feedback mechanism
Maybe I can use a switched resistor network 1ohm,2 ohm,4,8,16,32,64,128,256 ohm maybe R-2R ladder network(I dont know how can I use it in this application)
That was a great thread, with several conversations going on at once, and by the 15th you had a working circuit you were happy with, although it did have a multiplier. :-)
I made a copy this morning, edited for brevity, posted below. The multiplier solution comes near the end of the text, where Google put it, although it was one of the early forks.
Subject: Voltage Controlled Resistor? Group: sci.electronics.design
From: Tony Williams Date: Wed, Dec 11 2002 10:35 am Message-ID: Email: Tony Williams
What's a good way of doing a precision VCR? I thought it was easy, but have been doodling on paper for half a day now. I could have sworn it could be done without a multiplier. Must be going senile.........
The spec; Vcontrol= 0-10V, Rout= 0-1k, over the range 0-10v or 0-10mA. 15-0-15 available.
From: Jim Thompson Date: Wed, Dec 11 2002 10:53 am
I addressed this recently on A.B.S.E at... Message ID:
There's also a Voltage Variable Resistor part for PSpice on my website.
...Jim Thompson
From: fred bartoli Date: Wed, Dec 11 2002 12:04 pm
That kind of resistor made in POLY ???
From: Jim Thompson
Fred, I'll presume you're not already into the Christmas cheer and assume you mean POLYnomial and not POLYsilicon ;-)
You can do it with a polynomial, but PSpice has had, for several years, the EVALUE and GVALUE parts, thus you can avoid the gruesome polynomial notation.
From: fred bartoli
He he, I know that. Was just playing on the ambiguity...
From: Winfield Hill Date: Wed, Dec 11 2002 12:09 pm
It has scrolled off the servers now.
From: Michael A. Terrell
I have a copy if you want it.
From: Spehro Pefhany
Why not repost it to abse?
From: Michael A. Terrell
That was why I asked if he wants it. I knew I had downloaded it, but it took a while to find.
From: Tony Williams
Yes please Michael.
From: Michael A. Terrell
It has been posted to news:alt.binaries.schematics.electronic subject: Repost for Win Hill. If you can't get it there, I can E-mail it to you.
From: Jim Thompson Date: Wed, Dec 11 2002 7:09 pm
I sent Tony a copy via E-mail.
From: Michael A. Terrell
OK, I had posted it anyway. At least you know someone downloaded it. :)
From: Tony Williams
Received thank you. Still sitting here looking at it. 00:20 in the morning is not my best time.
From: Tony Williams Date: Thurs, Dec 12 2002 4:38 am
And I still can't see it. Because I can't get past the interpretation that Q2's emitter is forced to sit at -Vbe(Q1) permanently, by the opamp. So the range of Vcontrol looks limited to the Vbe difference between two transistors with their emitters joined.
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From: Winfield Hill Date: Thurs, Dec 12 2002 9:18 am
It's a small piece of an analog multiplier circuit.
The current in R1 and hence Q1 is I1 = Vin/R1, so Vbe1 from Eber-Molls is Vbe1 = VT ln(I1/Is), where VT = kT/q and Is is the reverse saturation current (which in a complete setup would ultimately disappear from our equations). Seeing that Vbe2 = Vc + Vbe1, we can determine I2 as a function of Vc, I2 = Is e^ (Vc/Vt + ln(I1/Is)). This can be manipulated into an I2 = I1 * K form, where K is an exponential function of Vc. So we see Rin = R1 in parallel with a programmable resistance.
Isolating R1's current from the IN terminal with a follower, and deriving Vc from a current into a logging transistor to fix the exponential-function issue, can clean up the control equations. Two more transistors and three more opamps later we'll obtain a resistance proportional to Vc looking like, R = (Vc/R2)/(Vref/R3). I'll post a new drawing and explain the equations tonight or early tomorrow morning; right now I have to walk the dog and go to work. :>)
From: Tony Williams - view profile Date: Thurs, Dec 12 2002 10:20 am
I'd already got as far as ln(I2/I1) = Vc*(q/kT) and therefore the actual incoming control voltage would have to be logged, such that Vc = kT/q * ln(Vin). This would give I2 = I1 * Vin.
And the easiest way to get rid of the non-linearity due to the (1 + Vin) term is to buffer R1, so that Iout does not have to include the I1 current to R1.
From: Winfield Hill Date: Thurs, Dec 12 2002 10:13 pm
Right.
OK, i'll indulge myself. Here's the full drawing and formula for R ~ control voltage. R_in = R1 (Vset/Vref)(R3/R2), if I avoided mixing up a numerator and denominator someplace!
If Q1-Q2 and Q3-Q4 are each matched to 25uV, and with a beta approaching 1000, the formula should be accurate to about 0.1% Alternately a few trim-point adjustments can be added.
Thanks, - Win
From: Tony Williams Date: Fri, Dec 13 2002 4:11 am
That topology (4x transistors, log/antilog config) is almost identical to the 'circuit' of the Raytheon RC4200 analogue multiplier...still available, very cheap, good linearity.
It would need an RC4200, two opamps, MOSFET and current shunt. Quite a simple circuit really.
Both approaches result in Rout = Vout/Iout, = K.Vset, but there is a subtle difference in the methods.
The topology above is of an output current device, setting Iout according to the sum; Iout = Vout/K.Vset. ie. It works by taking the reciprocal of Vset, so the circuit is at highest 'Gain' at low values of Vset.
Speff's topology is of an output voltage device, setting Vout according to the sum: Vout = Iout*K.Vset. ie, It requires a straight multiplier.
From: Winfield Hill Date: Fri, Dec 13 2002 7:47 am
. o x o y o out o z . _______| | | |_______ . | | | | | | . | |/ Q1 | | Q4 \\| | . gnd --|-----| ______| | |-----|-- gnd . | |\\V | | | V/| | . | | | Q2 |/ Q3 \\| | | . | _ +--|----| |-----+ _ | . '--|-\\__| | |\\V V/| |__/-|--' . V1 ---|+/ | _ |_____| \\+|--- gnd . '--|-\\____| . V2 ---|+/ Iout = Ix Iy / Iz
OK, I wonder if it would need that much stuff. Try this on for size. Apparently you'll need an R-C on each input pin.
. Rin = 40k Vset/10 . IN o-------------------------, . G =_+1 | . __ R1 ___/ |___| ____ R3 ___ Vref = 10V . | 40k \\_| | / 40k . | ____ | __/ . | | | ____ R2___ Vset . | | | | 40k 0 to +10V . o x o y o out o z . _______| | | |_______ . | | | | | | . | |/ Q1 | | Q4 \\| | . gnd --|-----| ______| | |-----|-- gnd . | |\\V | | | V/| | . | | | Q2 |/ Q3 \\| | | . | _ +--|----| |-----+ _ | . '--|-\\__| | |\\V V/| |__/-|--' . V1 ---|+/ | _ |_____| \\+|--- gnd . '--|-\\____| . V2 ---|+/ Iout = Ix Iy / Iz
Oops, I see the issue, you need higher output currents and a 1k rather than 40k range. I see, the extra opamp FET and shunt are so you can keep the currents below 250uA.
I wonder how much effort is required to get all the offsets right, etc., for 0.1% performance (fig 7). AD734, wink wink. Anyway, very nice, I had forgotten that part. Great that it's still available,
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And only $4.59 from Newark (stock # 34C1805 and 34C1803).
One thing I noticed, they used transconductance opamps in the RC4200, which is a good idea. Small series resistors at the opamp outputs would be a good idea in my circuit for loop stability at high currents. Also note, no V+ supply!
From: Tony Williams Date: Fri, Dec 13 2002 6:32 pm
Just to clarify, everything below the x,y,out,z line is inside the RC4200.
Yes. Note also that the Q3 collector output is limited to 5V above the 0v pin.
The Fig.7 in my data sheet is marked "Extended Range Divider and the large collection of resistors around the RC4200 is to do with obtaining a bipolar Vx and Vout.
As noted in another post, this particular topology does require taking the reciprocal of Vset (as per the use of the z input in your circuit above). This would raise concerns about the Vos of the z input when operating with low values of Vset.
Note though that the z input opamp does not have a Vos adjust facility.
I've used the RC4200 twice on jobs. Worked ok both times, no nasty surprises, all devices well within the tyical spec.
Trouble is it's one of those chips that grabs your interest, but the minor limitations/inconveniences reduces the number of job you can actually use it on.
-- Tony Williams.
From: W> W>> Oops, I see the issue, you need higher output currents and
Right, and 0.25mA max for linearity, got it. The buffer for the current-multiplying resistor, Rx, needs < 0.5mV offset, and Qx needs a stable beta value.
From: Spehro Pefhany Date: Wed, Dec 11 2002 11:11 am
Could you PWM an analog switch (or just a MOSFET) in parallel with a 1K00 resistor?
Best regards, Spehro Pefhany
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From: Tony Williams Date: Wed, Dec 11 2002 5:01 pm
I don't think so Speff. Linear if poss.
-------------------------------------------------
From: W> What's a good way of doing a precision VCR?
You can make 2-quadrant voltage-controlled resistors with a programmable transconductance amplifier (OTA) such as an LM13700. These are not very expensive and a 20:1 range is practical. OK, an OTA is a type of multiplier. So what's wrong with using a multiplier?
Hmm, do you only need single-quadrant operation? You could program a simple active resistor with an EEPOT.
From: Tony Williams Date: Wed, Dec 11 2002 5:10 pm
+10v Vref +-------+ | \\ / 1k(approx) \\ | ADC> First find a big well-matched dual FET ....
Is that based on the idea that drain-source resistance is almost linear with small gate voltages? (Not sure about which terminal is which)
From: Tony Williams Date: Mon, Dec 16 2002 4:30 am
It was based on pairs of jfets, selected so that their ON-resistances versus Vgs were matched. The yield of such pairs was quite low, but it was viable because we bought jfets by the bucketload, and charged an absolute fortune for the final product.
*Precision* 0-1K (0.1-0.01 Ohm accuracy? ) would not be very easy with a digital pot, directly anyway.
I did some work on this in terms of calibrator design some years ago, can't recall the conclusions at the moment, but it's certainly not insoluble. Might have used a multiplying DAC and some circuitry.
From: Winfield Hill Date: Wed, Dec 11 2002 1:22 pm
Whoa, 10-ppm! Maybe Tony'd better tell us more.
Yep.
From: T> I did some work on this in terms of calibrator design
Speff emailed me a few details. Below is a development of his ideas towards my spec.
/ | Iout +-[R1]--- 0v---|X2 | | R2 1k, 0.1%
I had a trace of difficulty understanding your drawing, so my schematic-editing compulsion took over...
Very nice, but use an AD734 and let the opamp do it's job driving 10mA, I say!
From: Tony Williams Date: Fri, Dec 13 2002 3:27 am
| \\_/D1 | D2 | _____ +------|>|------+ > | | | |
My app probably needs to prevent Vout going below 0v. So I've tucked in two extra diodes to do that.
I'm getting this aw(e)ful sinking feeling about AoE III, particularly Appendix E.................
Reducing the Vpk seen by the multiplier is quite a help to linearity. I've done measurements on the AD633 and keeping Vpk at no more than about 6 volts is handy.
Holdover Win, from the days when 5mA was the standard max output, and the habit was to avoid offset drift due to dissipation in the opamp package.
From: Winfield Hill Date: Fri, Dec 13 2002 8:27 am
If you use an AD734, watch out for the 50k Rin; followers can solve the issue, shown above. Perhaps these aren't necessary with an AD633, although there can be 2uA bias.
From: Tony Williams Date: Sun, Dec 15 2002 10:10 am
3k +15v----/\\/\\-------------+--\\/\\/\\--0v 1.5k | [Ammeter] for Iout. |
I breadboarded the above this morning.
R1= 33.378k, R2= 601, R4= 10.035k.
This was to give me 200 ohms/volt over a Vin range of 0-6V.
I've only had time to have a quick look, adjusting the Zero and Gain of the decade voltage source for 0 and 1200 ohms.
Vin Vout/Iout ~~~~ ~~~~~~~~~
0 0.0 measurements with better opamps,..........
If this circuit goes to layout then of course better components will be used (inc multiplier), but did a quick breadboard with what is available.
Ok, that AD633 is being (over)pushed, but not as bad as you think. ( Oh buggerit! .... see [*] below.)
The non-linearity of the Y input is 0.1% (0.4) for a 10-0-10V swing. So I use Y as the Vin, 0-6V max.
The non-linearity of the X input is 0.4% (1.0) for a 10-0-10 swing. With a spec range on resistance of 1000 to 250 ohms, the current only varies from about 5mA to 8mA, a 1.6:1 range. So the swing on the X input is not large. With a 601 ohm shunt that would be a swing from 3V to 4.8V only.
[*] Except that clever-clogs here wired it without looking at the data sheet and forgot that Y is the linear input on some multipliers and X on others. Modified below to suit the AD633.
3k
If this is simply to drive a gauge input then it may be simplified greatly if you make some measurements with a variable resistor between gauge and GND. Or just use 10, 22, 33, 47, 68, 82, 100, 120, 150, 180 ohm resistors, one at a time, record the voltage measurement across the resistor, that is from gauge terminal to GND; and note the gauge reading for each resistor, best estimate is good enough. Then report back with the tabulated results.
Do you know how the downstream circuit measures the 'resistance'?
It may simply be a current source looking at the resulting voltage - simply requiring a voltage input.
Could be an oscillator lookng for a resistance-set current - simply supply a current.
Wouldn't it be strange if the indicator turns out to need a 0-20mV input, or if your pressure sensng 'signal' originates in a 10 to 180 ohm transducer?
The FET is nice but nowadays it's hard to find one that can cover the ranges above 100ohms nicely enough. They are all geared towards low RDSon these days and thus have large die and huge capacitances.
If performance is only required above a few MHz I prefer PIN diodes. Currently I am wrestling with these again, fighting rectifying effects and all that, plus scouring the dreadfully slow Philips web site for a SPICE model of their BAP50. Mostly the site trundles "waiting for stockquote blah blah blah...". I mean, why don't they just place the sub-circuit or model statements at the end of the data sheet?
You missed my point... there's an OpAmp plus a series source R to set PRECISE emulation... sort of like a current mirror.
...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 |
It\'s what you learn, after you know it all, that counts.
Sure, I have used that as well in the past to obtain a linear range and/or precise setting. But it doesn't make the large capacitance go away :-(
Of course, I already know your answer. But I can't make my own devices, we discrete designers have to live with what the semi mfgs place into the vending machines.
Any chance that Lansdale will some day offer the 74HC equivalent of the CD4007 but with very low capacitances?
I doubt it. You could call Dale Lillard and ask. But Dale tends to build them just like they were in the old days, not as you'd like them to be today.
I just got an RFQ to reconstruct an obsolete AM/IF-strip/detector chip ;-) (Military use.)
...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 |
It\'s what you learn, after you know it all, that counts.
It would have to be at least like the SD5400 in RF performance. But the market would be small since most young EEs don't know how to design with this stuff anymore.
That's a surprise. Usually the defense folks insist on water-tight contracts with clauses such as the surrender of the mask plots in case of obsolescence or breach of delivery schedule so they can go out and have it fabbed elsewhere.
This is a strange one... ALL documentation has been tossed, though I did manage to scarf up a rough schematic.
...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 |
It\'s what you learn, after you know it all, that counts.
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