Can anyone shed some light on how multimeters, scopes and other similar devices can measure anything from microvolts to 300V or more without any moving parts (relays, etc.). Specifically, how do I design the front end for an A-D capable of measuring up to 100V without sacrificing performance at lower signal levels too much?
What about safety considerations? How do I isolate a 120V input from the user, especially when that same front end has to measure millivolts.
My application is a home brewed, flexible data logging/low freq oscilloscope device. I want to be able to handle reasonably high voltages (120V if possible), but at the same time be able to measure millivolt waveforms via a high gain ins-amp at the front end. What if I limit myself to something like 30V input, does that simplify things?
In the end, I need a high impedance input that's robust and can switch between mV measurements and V measurements without any moving parts (i.e. relays).
I think I'd like to get 8 bits of meaningful resolution at 1mV P-P, and at least 12, ideally 16 bits at higher signal levels (10V P-P). Bandwidth wouldn't need to be too high, ideally 100kS/s I'll be happy with 10kS/s.
I understand what you mean about the input protection. If I was limited to only a couple of volts, I'd just switch gain on an instrumentation amp, letting it clip to its heart's content if the signal got too high. But at 100V, it'd fry the whole thing.
I get the input resistor, the other thing I'm not familiar with. Assume for a second that I have little or no experience with the practical side of this type of electronics. Can you shed some more light on what you mean by low-leakage input protection, perhaps in terms of a circuit?
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things?
16bits at 10V P-P, maybe 8 meaningful bits (after figuring noise, etc) at 1mV P-P. If I attenuate the input, doesn't that inherently mean that I attenuate my, already small, 1mV signal too?
What is "too much"? There are a lot of games that you can play with attenuators (switching the low side, and relying to some extent on input protection / fixed input limiting resistor) that fall apart when you go from essentially DC to a much wider bandwidth.
Input resistor and low-leakage input protection.
What's the highest resolution that you will need? Can you attenuate the input, and have variable gain thereafter?
Bandwidth? Resolution/accuracy for smallest inputs?
There are some really cool optically coupled semiconductor "relays" (NAIS) but most of these have somewhat large leakage and capacitance for these applications. The last I knew 'scope makers were still using relays :( {I would be happy to hear if this information is out of date, especially if that included how it was done...}
Hmm. I bought an older ('93) Iwatsu DS6121 DSO from military surplus. It works well and has a series of small relays inside that switch the active preamp. They barely even make a "click" sound. (It also has two slots full of "74F" series logic chips comprising the RAM - could probably save a couple hundred watts if they were changed to CMOS!)
There are several ways to accomplish "auto-ranging", here's one idea. I have not made this device nor tested it, so take these words with a big grain of salt. It is also not linear in any way. It is just an idea to get you thinking. Perhaps it is useful, perhaps it is not.
"Smart" electronics usually require the use of a processor somewhere, and this is such an idea. Imagine that an input voltage is divided between "taps" on a resistor ladder and the voltage is read relative to that divisor. i.e.,
Those values can surely be tweaked to provide better numbers, but frankly I ain't got that kind of time.
Now you would run all these taps to something like a 4066 digital switch (provided one of these can handle at least +15v in the "on" condition, but they may not like +155v in the "off" condition - please check its datasheet. A series of opto-isolators and transistor "switches" might work in place of the 4066.)
Okay before one gets too confused, here's what I'm getting at. For a
155v DC input voltage:
Tap 0 = 155.00v Tap 1 = 75.00v Tap 2 = 35.00v Tap 3 = 15.00v Tap 4 = 5.00v
Your device reads tap 4. It is within 0-5v (at 5.00v.) Your device reads tap 3. It is outside 0-5v, testing halted. Tap 4 = "multiply value read by 31 to get volts present" 5.00 * 31 = 155.00v
For a 32.0v input voltage:
Tap 0 = 32.00v Tap 1 = 15.50v Tap 2 = 7.22v Tap 3 = 3.10v Tap 4 = 1.03v
Your device reads tap 4. It is within 0-5v (at 1.03v.) Your device reads tap 3. It is within 0-5v (at 3.10v.) Your device reads tap 2. It is outside 0-5v, testing halted. Tap 3 gives higher resolution than tap 4, so use that. Tap 3 = "multiply value by 10.32258065 to get volts" 3.10 * 10.32258065 = 32.00v
For a 8.55v input voltage:
Tap 0 = 8.55v Tap 1 = 4.14v Tap 2 = 1.93v Tap 3 = 0.827v Tap 4 = 0.276v
Your device reads tap 4, 3, 2, 1, and stops at 0. Tap 1 gives highest resolution, so use that. Tap 1 = "multiply value by 2.065217391 to get volts" 4.14 * 2.065217391 = 8.55v
Now if you wanted mV resolution and below, turn on an analog x2 or x4 multiplier at tap 0 (as long as tap 0 voltage is /2 or /4 VanalogVcc.)
For that matter, instead of "peeking" and "poking" a ladder this way, a better idea might be to "R2R ladderize" the feedback loops of a division-mode op-amp and a multiplication-mode op-amp connected in series. Say a 4-bit R2R ladder for each, so an 8-bit microcontroller can control the whole shebang with one port. (i.e., high nibble = multiplier amp, low nibble = divider amp, cycle through all range combinations until measurement closest but under 5.00v is found, then compute volts * (all multipliers) / (all divisors).)
Hmm, calibration sounds tricky though. And temperature stability- ehhhh! :)
Again, just ideas. Besides, experimentation is good for you.
P.S. most "C" compilers for microcontrollers can do floating-point math. If C isn't your thing, may I suggest a peek at Ziya Erdimir's exponent-mantissa port to JAL,
formatting link
and my handy-dandy JAL floating-point-to-LCD-formatting front-end,
formatting link
-- "One day, personal computers are going to be labeled just as addictive as narcotics." MCJ 200405
Ouch! Let's assume for the moment that you need an input R of 1megohm (standard for 'scope), and that the 100kS/s is roughly comparable to
50kHz bandwidth. Just to make things really simple. This translates to a noise voltage (at room temp) of ~27uV rms, assuming that your electronics is perfectly noiseless -- this just including the source resistance. 8 bits with 1mV p-p translates to a step size of ~4uV. That's pretty absurd if you're looking for 'scope-like operation.
You're not going to get there without changing something pretty drastic. If you can narrow the bandwidth (a lot) you might get somewhere; you can't lower in input resistor much without getting into trouble, unless you use relays and decrease the input impedance of your device.
Of course, there is a more complex and expensive way of doing this. If you were willing to put in a high voltage voltage-follower on the input, you could do some cute things with some "high voltage" diode bridges at relatively low impedances. If cost is no object this might be possible. (I actually have done something similar for a special biomedical app). A relay or two is much cheaper.
I think (without actually looking at any DMM schematics) that they rely on an initial fixed input resistor, and do all their switching closer to ground. That way the switches don't have to operate at high voltages. The penalties are twofold: (1) input protection leakage currents must be very small, since they will contribute substantially (I*R) to errors; and (2) you don't benefit from low source resistances to reduce the noise when the signals aren't so big. Of course, with a 'scope-like input I assume that you don't want the input capacitance to be large, either, unlike DMMs!
I suggest you rethink your specs, especially why you cannot tolerate relays.
Sure, use ohms law and a resistive divider network.
Imagine 2 resistors in series, a 10 Megohm and a 100 ohm. The total is about 10M
Ohms law approximates voltage EMF to be = current (I) x resistance (R) and if youre hooked into 300V, with 10M then you need to find I which is I = E divided by R, so 300/10M I think thats 3E-5 Amps (3E2/1E-7)
Or take the ratio of 10M/100 which may be 100,000 to 1, if you read across the 100ohm resistor its a small voltage
Now you can tailor the divider in such a way to get the input swing voltage on your measurement device
DVMs have big manually-operated rotary switches, as do analog scopes. Most digital scopes do in fact have relays; you can hear them click as you change vertical ranges.
Perhaps it's time for me to spend some money on a multimeter and a good pry bar...
Thanks for all the posts, they've given me a bit to work on. In general, I think I'm going to end up making special gain stages separate from the main DAQ. Connectors are cheap and it won't take much to snap on a high gain or high attenuation module to an otherwise ordinary +/-10V DAQ front end.
I think this part of the issue is the part that the OP was asking about, and I don't think any of the respondents really addressed it. Obviously a 4066 would fry.
My Fluke meter has no relays (and damn few of any type of components). It does have what looks like a resistor network on a 1cm x 4cm substrate with several taps, but nothing that looks like a solid state relay. There are several SMT transistors, but only one or two TO92s.
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Big resistors. That's easy for DC or low-frequency AC. Scopes are wideband and can't tolerate large unbypassed resistors, so need switched dividers. Solid-state switches still have too much capacitance and can't tolerate overvoltages, so most scopes still use a relay or two in the sront-end.
Sure, but "big resistor" isn't so sensitive to "small switch resistance" and "low bandwith" doesn't have to have the high capacitance of the FET switches compensated for. It's all about bandwidth.
I'm sorry I haven't been very explicit about what has me baffled. What I don't understand is how those small switches can take the high voltage sometimes present. For low ranges I think there has to be a switch near the top of the divider, right? If so, why doesn't it get fried when there's a high-voltage input?
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I don't believe the switches on a DMM are on the input side, rather in t he feedback of an instrumentation amp. It would hard to maintain a multi-megohm input resistance with a resistor divider hanging on.
Someone who is more knowledgeable (has ripped one apart;) could better comment here. You're right though the few hundred volt input tollerance is impressive.
The switch never sees more than a few volts or a few tens of uA.
Best regards, Spehro Pefhany
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If switch Sw is open (for low-voltage signals), there's no attenuation. Sensitive operation. If Sw is closed, you get attenuation. Sw never has to see the full input voltage. To protect the ADC measuring Vo, you need a low-leakage protecting device of some kind. The source is protected from the clamp by R1.
I think that's the best idea yet. Perhaps replace the switch entirely with a tri-state device? (assuming the parasitics aren't going to munge the rest of the circuit.)
The uC has the job of keeping the divider at the highest range, until a signal change is detected, then it ramps down until the reading is within range. Perhaps it can detect high voltages without conduction by electrostatic means? (A FET gate running parallel to the input trace --> latch --> timer?) No, that wouldn't work when in the highest range and a small voltage were applied. Maybe then there is a high-impedance frequency generator imposed on the divider, and the amplitude delta triggers ranging? Or for that matter, what is going to be able to measure 400mV at Z=10MOhms in the 400v scale? We need a pro's answer. :)
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