So I have this project that uses a dsPIC, in this case a DSPIC33FJ256GP506. Not my choice, and I wouldn't have used it in the first place but ...
When I wanted to use the ADC I looked around in the datasheet. Not much there to write home about.
On the bottom of page 289 in the datasheet it says under "recommended impedance of analog source voltage": 200ohms max. Ohms! Is this a joke? What were they thinking?
-- Regards, Joerg http://www.analogconsultants.com/
A lot of high-speed ADCs seem to have roughly similar requirements. The Analog Devices PulSAR ADCs are shown (in some of AD's papers) as being drive by either transformers, or dedicated high-speed driver chips, with perhaps 35 ohms in each leg of the differential drive feed.
I imagine that this level of studly low-impedance drive is required to allow the ADCs to sample at high rates... the current to charge and discharge the sampling capacitors at megasample rates isn't small.
If you're working with low frequency ranges and low sampling rates, you might be able to use a higher driving impedance, and use a stabilizing capacitor right by the ADC input pin (basically a low-pass filter arrangement).
--
Dave Platt AE6EO
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That's what I put in there. Essentially it's measuring voltages and I didn't want to place expensive low offset opamps. The cap puts the lowpass around 1kHz, should be enough.
I was surprised how little information there was in the datasheet. 320 pages and no meat for the ADC. On the chip itself things are rather spartan as well. They didn't even spring for a bandgap. At least the datasheet doesn't mention any.
-- Regards, Joerg http://www.analogconsultants.com/
It's no joke. The low impedance allows for the fast sampling rate compared to the 8-bit PIC's
Microcontroller data sheets are often very rich in detail on the digital parts, and skimpy on the analog.
Presumably it's because when their mixed-signal consultant turns in his netlist and says "Design's complete; it still needs to be verified and tested when you get first silicon, and when do you want my input on the data sheet" the project manager says "fine, we'll call you then" and then never does.
-- www.wescottdesign.com
It's probably a C-2C chain, so when it samples it's effectively switching in a capacitor with an unknown voltage (well, dependent on the previous sample, at least). They call out the low impedance to make sure that the transient has settled by the time the sampler switches off and the ADC starts shuffling the capacitive ladder around.
If it's like the ones that I've worked with the recommended way of dealing with it is to either use a circuit that's really low impedance, use a circuit that's fast enough to recover within the sampling interval (which is getting harder and harder as time goes on), or put a honkin' big cap right at the ADC terminals and stabilize it (and make sure that all that oddball current injection doesn't do nasty things to your accuracy).
Fun stuff. It's why analog circuit engineers are still gainfully employed in this degraded, "all digital" world that we live in.
-- www.wescottdesign.com
Microcontroller data sheets are very poor on any kind of details. As soon as you start digging deeper then "hello world", you find out this. The usual answer from support is "take it from schematics of evaluation board" or "look it in the source code which comes with evaluation board".
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
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I just checked the PIC32, since it's from the same company. However, it allows upto 5Kohms for sampling upto 400KSPS. For 400KSPS to
1MSPS, they suggested 500 ohms (sliding scale?). It is still much better than the dspPIC33.
Just yesterday we made a discovery. On a new 64-channel scanning ADC module we're developing, we use an AD7699 16-bit mux'd 8-channel ADC, scanning a tree of upstream multiplers. Each ADC input is driven from a PGA through a resistive voltage divider, with a cap across the divider output.
We were seeing a parabolic nonlinearity on the order of 0.03%, origin mysterious. So we added a 2nd order polynomial correction in the FPGA, driven from a cal table, with every channel curve-fitted for zero, slope, and 2nd order. That fixed the bend, but there was still a DC offset that the cals corrected but didn't make sense.
Turns out that if we remove the caps, everything gets beautiful... no curvature, no big offset. So we can ship today.
As long as all that polynomial stuff is in there, we'll just leave it. Can't hurt.
So, the capacitance seen by a sampling ADC can do weird things.
I've also seen opamps go nuts when whacked by the charge injection from an ADC, which is one reason we used the divider and cap.
John
Ok, but they could have been a wee bit more detailed. Many times all you want to do is monitor the voltage on some lines, say, from potmeters. You don't want 200ohm potmeters and ideally also no buffer amps that would introduced additional offset errors. They change slowly. So one would need to know the DC input impedance and then slap a cap across it so the ADC is happy. But they fail to state a DC input impedance.
-- Regards, Joerg http://www.analogconsultants.com/
The dsPIC33F family reference manual section 16 provides 66 pages of documentation on the ADC and the 1st paragraph in the ADC section of the datasheet tells you to look at it.
So where does it state the DC input impedance for the DSPIC33FJ256GP506?
-- Regards, Joerg http://www.analogconsultants.com/
I find that to be a problem with most datasheets now. At one time, datasheets had schematics equivalent circuits of I/Os, if not the entire device (admittedly a little tough for a micro ;-).
They did have that for micros as well. Some mfgs do, some don't. All I usually need to know is the first 2-3 devices behind a port pin.
-- Regards, Joerg http://www.analogconsultants.com/
You probably meant multiplexers?
As code, but not used?
Interesting.. I have a project (still on the drawing board) that uses this device. Do either the offset or the curvature depend on the channel sequencing, or what signals are on the other channels? -f
h ?
I have a feeling that the 200 ohms max is just recommended value at high sample rate. I have a 10K resistor between the battery and A2D, although they recommended 5K. My readings are accurate enough.
True, but the ancillary documentation is often quite complete. I've never flogged a dead processor for days without ultimately finding the one improperly set up "screw the processor bit" in some obscure section of some manual dedicated to some apparently totally unrelated section of the processor.
(Although it only took me about 15 years to realize that I need to read EVERY WORD of any document that purports to explain how the system clocks are distributed).
-- www.wescottdesign.com
If it's a switched-capacitor ADC there probably isn't a DC input impedance per se. Most likely the closest thing to a "DC input impedance" is the effective series resistance of the switch in the sampler, and that's most certainly not going to be on all the time.
-- www.wescottdesign.com
That's also what I think. But "likely" doesn't fly for some designs :-)
IME when you call in you get an answer but many times not in writing. For many of my projects I do need it in writing, and it has to be something that was properly ECO-released as a document. There have been many parts during my career that I could not design in because of this. Sometimes whole series of parts.
-- Regards, Joerg http://www.analogconsultants.com/
I also think it'll work. In this case it isn't safety critical, it is just so we don't deplete a rechargeable battery too far. There is a secondary shut-off and even if that fails, it isn't a LiIon.
More like a convenience message to the users in my case, along the lines of "Hey, you've only got enough juice for this many sessions".
-- Regards, Joerg http://www.analogconsultants.com/
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