Output resistance modulation - design/approach ideas? - Page 1
Forum section: electronics / beginners

author: Chris_PL
author_rank: Contributor
timestamp: December 08, 2013, 07:49:31 pm
content: HiI'm in need to modulate the output resistance of an analog signal just a little bit (few Ohms) using sine or arbitrary pattern from a DAC. It's a ECG signal simulator circuit (which works perfectly) and I need to add simulated respiration signal to it - which, basically is a minimal output resistance variation due to chest movement and lungs expansion. To explain things more presicely - here's the DaveCAD drawing:(Yes, it has a path to the ground after the op-amp buffer - forgot to include a voltage divider on the drawing)I have a few ideas how to do it, but when it comes to practical design I'm not sure how to proceed.1) Discrete components. A standard 5mm LED driven from voltage-controlled current source, epoxy-sealed with photoresistor to eliminate ambient light. Something like this:Problems:a) ~ 1kOhm is the lowest resistance I could go down to, when using an off-the-shelf jelly-bean photoresistor and a reasonable LED current (i.e. =< 20mA)b) I have no idea if it is even possible to make it work with so small resistance changes.2) More "digital" approach. Digi-pot.Problems:a) Lowest pot value you can easily obtain is around 1kOhmb) It is not "isolated" - measuring respiration from a patient monitor point of view needs high-frequency, modulated current injection, so using a digi-pot doesn't sound like a good idea.3) I've heard something about using a FET in its Ohmic region to act as a variable resistor, but don't know completely how to start, and even if it is a good way to do it.Am I missing some obvious and easy method?PS. No, it is not a homework assignment. I'm a electronic hobbyist, who works in a hospital lab for a living. Just trying to have fun with electronics somewhat related to my professional work (although completely from the other side - I'm using patient monitors and medical/biological stuff at work, not testing them)

author: Marco
author_rank: Super Contributor
timestamp: December 08, 2013, 08:31:47 pm
content: A FET or photoresistor can work, but not perfectly linear and/or will drift with temperature ... what's the bandwidth of the source? You might be able to use a variable load circuit.

author: Chris_PL
author_rank: Contributor
timestamp: December 08, 2013, 08:53:16 pm
content: Quote from: Marco on December 08, 2013, 08:31:47 pmYou mean the bandwidth of the output signal? 0.1 - 100Hz, it's an DAC-generated ECG signal w/low pass filter.Resistance "modulation" signal is generally in sub-Hertz range, using a 256-step sine approximation.

author: Marco
author_rank: Super Contributor
timestamp: December 08, 2013, 09:13:49 pm
content: Oh wait nm, I wasn't thinking straight the variable load circuit doesn't solve the problem. I think a variation of this circuit would work for you :http://www.edn.com/design/analog/4368893/Use-a-photoelectric-FET-optocoupler-as-a-linear-voltage-controlled-potentiometerUsing the second photofet for feedback removes the non-linearity of voltage/resistance (when the devices match and the article suggests they match reasonably well). Something you couldn't do with a DIY led/photo-resistor pair.

author: Chris_PL
author_rank: Contributor
timestamp: December 08, 2013, 09:31:08 pm
content: Thanks! Seems worth trying. But wouldn't the FET distort my low voltage, already filtered biosignal (5mV p-p max, more like 1mV p-p) too much?

author: Andy Watson
author_rank: Super Contributor
timestamp: December 08, 2013, 09:44:43 pm
content: Could you afford to use discrete steps of resistance, i.e. switch in/out resistor values using photo-mos type relays?Or, if your bandwidth of interest is limited to 100Hz, how about using a switched capacitor configuration. Switching at >>1kHz, the capacitor looks like a resistance that is proportional to frequency of switching.Edit: Make that inversely proportional to the switching frequency.

author: Chris_PL
author_rank: Contributor
timestamp: December 08, 2013, 09:55:54 pm
content: Quote from: Andy Watson on December 08, 2013, 09:44:43 pmYes, I thought about it at the very beginning. I wanted to use mini reed-relays (which I have plenty, in the junk parts bin), but to make a decent "resistance change sine wave" I'd have to use at least 16 of them (am I right?) and even then it'll be more a staircase than a smooth change.When I look at the patient monitor output with a Fluke Tester connected to it, the respiration wave is smooth as hell, so there has be a decent way to do it. Unfortunately I couldn't find any patient simulator service manual w/schematic to see how it is done in a proper way. Probably they make their own LED-photoresistor custom chips or sth.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 09, 2013, 12:16:59 am
content: The signal is purely AC, right? It's not shifted up?

author: Chris_PL
author_rank: Contributor
timestamp: December 09, 2013, 12:23:43 am
content: Quote from: minime72706 on December 09, 2013, 12:16:59 amNo, signal to be resistance-modulated is not shifted. ECG DAC is powered from bipolar supply with "isoelectric line" set on 0V and everything in the analog signal path (filter, voltage divider, buffer) has bipolar supply.And the signal to drive mysterious "signal-to-resistance" device can be purely AC, DC shifted - whatever works best.

author: minime72706
author_rank: Frequent Contributor
timestamp: December 09, 2013, 01:04:26 am
content: Quote from: Chris_PL on December 09, 2013, 12:23:43 amThough I've never tried it, partly because it takes a bit of tuning, you can use MOSFETs as variable resistors if you bias them correctly. I believe JFETs are known to perform this role better. I recently grabbed a couple "voltage controlled resistors" in case I ever need such a thing. An example of an application other than yours would be in an automatic gain control circuit in a radio.

author: Rufus
author_rank: Super Contributor
timestamp: December 09, 2013, 02:44:17 am
content: If you create your 500 ohms output impedance from a voltage divider (say 1k + 1k) you could put a ground referenced FET in series with the bottom resistor to adjust output impedance with less sensitivity. The output level will change a little but if you are generating it all digitally you can compensate for the change digitally.

author: Marco
author_rank: Super Contributor
timestamp: December 09, 2013, 04:12:33 am
content: Quote from: Chris_PL on December 08, 2013, 09:31:08 pmA little, but at small Vds it's pretty linear.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: December 09, 2013, 07:45:55 am
content: Since you mentioned up to 1 Hz modulation frequency...How about a more "mechanical" solution?Use a dual-gang rotary pot driven by a DC motor.Use the first gang as the variable series resistance element. Fully floating!Use the second gang in a voltage-divider configuration as an angular position (which linearly corresponds to output resistance) feedback signal. You can then control the DC motor with a PID loop. The setpoint for the PID loop is the input modulation signal.To keep the noisy motor away, you can drive the pot with a long shaft.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 09, 2013, 07:50:35 am
content: The effective output resistance of an op-amp is very low because of the combination of high gain and negative feedback. Could you artificially limit the gain of an op-amp by configuring it as a differential amplifier with finite gain, then use that block in a negative feedback configuration to create an amplifier with low, but non-zero, output resistance?It doesn't get you away from needing a programmable resistance, but that resistance is now setting the gain of an amplifier rather than being the actual output resistor itself. For this you could use an inexpensive digital potentiometer, which combines the functions of DAC and programmable resistor in one IC.

author: John Coloccia
author_rank: Super Contributor
timestamp: December 09, 2013, 02:41:08 pm
content: I'm still not sure I understand the application. Could you explain what the circuit needs to do and why? It would be better if we understood the application.edit:nevermind....I found it. If all you want to do is modulate the signal a little bit, why not just send the DAC and the modulated signal you want to a summing junction? Or does that not satisfy what you want to do?

author: trackman44
author_rank: Regular Contributor
timestamp: December 09, 2013, 03:11:12 pm
content: What are BJT transistors known for? Anyone? " transfer of RESISTANCE". You'll probably need aPNPwith low gain, so you can control the CE resistance better. Worth a try.WillEdit: Sorry, I meant to say NPN. Though, it could work with PNP, depending on how you implement your circuit.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 09, 2013, 03:48:01 pm
content: The datasheet for the LM 13700 claims a single-ended voltage controlled resistor application. It's not obvious how you would merge it with your ECG signal - perhaps via the other "end".

author: Rufus
author_rank: Super Contributor
timestamp: December 09, 2013, 04:18:00 pm
content: Quote from: John Coloccia on December 09, 2013, 02:41:08 pmHe wants to modulate the source impedance of the output a little bit. The effect of which depends on the attached load impedance and to be a realistic simulator he probably can't make any assumptions about what the load impedance is.

author: John Coloccia
author_rank: Super Contributor
timestamp: December 09, 2013, 04:27:10 pm
content: Oh, I see. He wants to use this to feed an actual ECG, not just simulate the signal. I missed that.

author: Chris_PL
author_rank: Contributor
timestamp: December 09, 2013, 04:49:34 pm
content: Quote from: Rufus on December 09, 2013, 04:18:00 pmExactly. I can't rely on any load impedance, because it is on the "monitor" side and differs between manufacturers, patient monitoring systems etc.I've been digging through the net today and I've found two service manuals for Patient Simulators. Unfortunately both use voltage controlled current source (w/feedback) and custom matched LED-photoresistor pair to do the thing. It seems that it's the proper method.Schematic excerpts:

author: Rufus
author_rank: Super Contributor
timestamp: December 09, 2013, 05:29:44 pm
content: Quote from: Chris_PL on December 09, 2013, 04:49:34 pmThe top circuit inserts a resistance of around 5k4 to 8k1 for a 2v to 0v control input. Seems a long way off the 500R in your DaveCAD circuit.

author: Chris_PL
author_rank: Contributor
timestamp: December 09, 2013, 05:57:24 pm
content: Quote from: Rufus on December 09, 2013, 05:29:44 pmYup. In the top design it is a part of larger divider network:But in the bottom design the resistance is injected just before output, after the dividers/attenuators :

author: Marco
author_rank: Super Contributor
timestamp: December 09, 2013, 10:52:05 pm
content: Quote from: Chris_PL on December 09, 2013, 04:49:34 pmThe article I linked earlier said the photofets they tried matched within 3%, which would seem close enough.

author: hlavac
author_rank: Frequent Contributor
timestamp: December 10, 2013, 12:37:05 pm
content: Photoresistor in parallel with the 500 Ohm output resistor looks like a good solution to me. Illuminate with sine-modulated current driven LED or small lightbulb.