passive component tolerances for op amps used in front end of 18-bitADC

[Nicholas Kinar]

Hello--

I'm currently selecting resistors for the feedback loops of a few op-
amps and in-amps used in the analog front-end of an 18-bit SAR ADC. The
ADC reference is 5V, so the smallest theoretical voltage (with 0V to 5V
input range) that can be resolved is 5V/(2^18) = 1.9E-5 volts.

Page 719 of the the "Data Conversion Handbook" edited by Walt Kester
gives an excellent overview of how resistor temperature coefficients can
affect ADC errors. According to an analysis given in this book,
resistors with a TC of 25 ppm/degC can be unsuitable for a 16-bit system.

Now it would be possible to eliminate some error by using a resistor
network with closely matched resistors. I would like my circuit to
maintain 18-bit accuracy over a temperature range from -40 deg C to 10
deg C, which is a 50 degree range. Since the resistors are matched, the
values will change in a similar fashion over temperature.

I think that I might have to use extremely high accuracy parts (which
are also extremely expensive), but I am hoping that there would be some
other way to maintain accuracy.

Could anyone recommend a manufacturer or source of resistors and
capacitors which could be used in an 18-bit system? Vishay is the first
company that comes to mind, but which series of components should be
used?

 

[Bill]

Hello--

I'm currently selecting resistors for the feedback loops of a few op-
amps and in-amps used in the analog front-end of an 18-bit SAR ADC. The
ADC reference is 5V, so the smallest theoretical voltage (with 0V to 5V
input range) that can be resolved is 5V/(2^18) = 1.9E-5 volts.

Page 719 of the the "Data Conversion Handbook" edited by Walt Kester
gives an excellent overview of how resistor temperature coefficients can
affect ADC errors. According to an analysis given in this book,
resistors with a TC of 25 ppm/degC can be unsuitable for a 16-bit system.

Now it would be possible to eliminate some error by using a resistor
network with closely matched resistors. I would like my circuit to
maintain 18-bit accuracy over a temperature range from -40 deg C to 10
deg C, which is a 50 degree range. Since the resistors are matched, the
values will change in a similar fashion over temperature.

I think that I might have to use extremely high accuracy parts (which
are also extremely expensive), but I am hoping that there would be some
other way to maintain accuracy.

Could anyone recommend a manufacturer or source of resistors and
capacitors which could be used in an 18-bit system? Vishay is the first
company that comes to mind, but which series of components should be
used?

Another option (which might be cheaper, although not simple) would be
to use an explicit temperature sensor to tell your system how much
error it should compensate for. Add some calibration, with maybe
polynomial interpolation. You wouldn't need much accuracy in sensing
the temperature. I don't know, I've never tried it, but it could work.

 

[Nicholas Kinar]

Another option (which might be cheaper, although not simple) would be
to use an explicit temperature sensor to tell your system how much
error it should compensate for. Add some calibration, with maybe
polynomial interpolation. You wouldn't need much accuracy in sensing
the temperature. I don't know, I've never tried it, but it could work.

That's a good idea, Bill! It would be possible to temperature-cycle the
PCB in an environmental testing chamber and add calibration for known
errors.

Obviously, I need to have some sort of "reference signal" against which
the system is calibrated. This could be provided by a function
generator and defined at a certain temperature. The trick is finding an
appropriate reference signal. Obviously, calibration would be
device-specific.

 

[Nicholas Kinar]

Could anyone recommend a manufacturer or source of resistors and
capacitors which could be used in an 18-bit system? Vishay is the first
company that comes to mind, but which series of components should be used?

The Vishay custom-made resistor networks look very interesting:

http://www.vishay.com/docs/63029/vsm4256.pdf

I would wonder how expensive it would be to create such a resistor
network with a TCR of <= 0.47 ppm.

 

[Nicholas Kinar]

Thank you so much for your response, John!

Susumu makes 15 PPM resistors, available from Digikey for around 35
cents each. We tested 200 ohm and 390K parts, saw -8 and +5 PPM
respectively. You can buy better resistors, well below 1 PPM, but
they're expensive.

It's neat to realize that the part performs better than the specifications!

Some esoteric resistors seem very much suited for military or other
hi-rel applications. Definitely not for mere mortals, and extremely
difficult to source.

You can get affordable 1 PPM/K voltage refs, too.

If you are willing to temperature cycle, you can measure the temp and
do software calibrations, but that's expensive too.

You can also occasionally mux in a known zero and a known 1 PPM-class
reference, and software calibrate continuously, fixing all the errors
except linearity. Then you can use junk 25 PPM resistors.

That's a great idea, John! So suppose that the 1 PPM-class voltage
reference is 2.5V. Since this is a known value, measuring the
difference between 2.5V and the voltage at a given temperature would
give the change in voltage caused by the change in temperature. But how
would I relate this to changes in resistance? I would suppose that the
change in voltage would be related to a change in the voltage gain of
the op amps comprising the analog front-end.

Would I have to switch the mux at every sample collected by the ADC, or
could I simply calibrate at the beginning of each measurement? I would
say that the calibration could be done at the beginning of every
measurement.

Too bad people don't make SAR ADCs with that quality of PGA built in.
I could sure use some about now.

Have you seen the AD7262 from Analog Devices? It only has a resolution
of 12-bits, but perhaps it might be interesting?

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7262/products/product.html

 

[Bret Cannon]

Susumu makes 15 PPM resistors, available from Digikey for around 35
cents each. We tested 200 ohm and 390K parts, saw -8 and +5 PPM
respectively. You can buy better resistors, well below 1 PPM, but
they're expensive.

You can get affordable 1 PPM/K voltage refs, too.

If you are willing to temperature cycle, you can measure the temp and
do software calibrations, but that's expensive too.

You can also occasionally mux in a known zero and a known 1 PPM-class
reference, and software calibrate continuously, fixing all the errors
except linearity. Then you can use junk 25 PPM resistors.

Some delta-sigma ADCs have a wide-range PGA on the front end that's
better than anything you can make. They have a near-zero drift
relative to the voltage reference. But slow.

Too bad people don't make SAR ADCs with that quality of PGA built in.
I could sure use some about now.

John

Where can you get resistors that are better than 1ppm? While I've worked
with resistors that are specified as better than 1 ppm/°C, about the best
stability I've seen are resistors like the Tinsley reference resistors that
are specified as 2 ppm/year
(http://www.tinsley.co.uk/products/standard-resistors/5685.htm) and they are
over $1k each and only available in values of 1 ohm, 10 ohms, 25 ohms, 100
ohms, 1kohm and 10 kohm.

Bret Cannon

 

[Nicholas Kinar]

Where can you get resistors that are better than 1ppm? While I've worked
with resistors that are specified as better than 1 ppm/°C, about the best
stability I've seen are resistors like the Tinsley reference resistors that
are specified as 2 ppm/year
(http://www.tinsley.co.uk/products/standard-resistors/5685.htm) and they are
over $1k each and only available in values of 1 ohm, 10 ohms, 25 ohms, 100
ohms, 1kohm and 10 kohm.

I believe that Digikey has these resistors:

http://www.digikey.com/

Example part number: Y1624-1KCT-ND

 

[Nicholas Kinar]

But how
would I relate this to changes in resistance? I would suppose that the
change in voltage would be related to a change in the voltage gain of
the op amps comprising the analog front-end.

Ah, I think that I've figured it out, John. It's simply another
calibration. The voltage difference between actual and calculated
reference is then subtracted from each sample taken by the ADC.

 

[Nicholas Kinar]

I believe that Digikey has these resistors:

http://www.digikey.com/

Example part number: Y1624-1KCT-ND

Apparently the temperature coefficient is listed as ±0.2ppm/°C for this
particular part number.

 

[John Larkin]

Thank you so much for your response, John!



It's neat to realize that the part performs better than the specifications!

Some esoteric resistors seem very much suited for military or other
hi-rel applications. Definitely not for mere mortals, and extremely
difficult to source.


That's a great idea, John! So suppose that the 1 PPM-class voltage
reference is 2.5V. Since this is a known value, measuring the
difference between 2.5V and the voltage at a given temperature would
give the change in voltage caused by the change in temperature. But how
would I relate this to changes in resistance? I would suppose that the
change in voltage would be related to a change in the voltage gain of
the op amps comprising the analog front-end.

You could mux in zero volts, digitize, then select the good vref and
digitize. From that, calculate a zero offset and a gain cal factor,
and apply them to actual input measurements.

Would I have to switch the mux at every sample collected by the ADC, or
could I simply calibrate at the beginning of each measurement? I would
say that the calibration could be done at the beginning of every
measurement.

You'd need to run a cal sequence often enough to catch any temperature
drift errors. Every minute or so would be OK in a benign environment,
where radical temperature transients aren't expected. More often if
possible.

Have you seen the AD7262 from Analog Devices? It only has a resolution
of 12-bits, but perhaps it might be interesting?

http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad7262/products/product.html

That's very nice. I want it in 16 bits!

John

 

[Nicholas Kinar]

You get less roundoff error by multiplying by zero and adding what it
ought to look like.

Yes, that makes sense. Thanks, Phil!

 

[Nicholas Kinar]

You could mux in zero volts, digitize, then select the good vref and
digitize. From that, calculate a zero offset and a gain cal factor,
and apply them to actual input measurements.

Sounds good, John.

You'd need to run a cal sequence often enough to catch any temperature
drift errors. Every minute or so would be OK in a benign environment,
where radical temperature transients aren't expected. More often if
possible.

In my experimental application (environmental ground-based remote
sensing), measurements are often taken on the command of a datalogger,
which sends an instruction to the measurement device across an SDI-12,
serial or CAN-bus. So as long as the calibration occurs immediately
before taking a measurement or within less than a minute, it should be
okay.

That's very nice. I want it in 16 bits!

It would be great to find such a version of this part in 16-bits!

 

[Nicholas Kinar]

You could mux in zero volts, digitize, then select the good vref and
digitize. From that, calculate a zero offset and a gain cal factor,
and apply them to actual input measurements.

You'd need to run a cal sequence often enough to catch any temperature
drift errors. Every minute or so would be OK in a benign environment,
where radical temperature transients aren't expected. More often if
possible.

Thanks, John! This ought to get me going with respect to the analog
front-end design.

 

[Rich Grise]

Phil was teasing. He does that sometimes.

Isn't that how the IPCC climate models work? ;-)

Cheers!
Rich

 

[Nicholas Kinar]

Isn't that how the IPCC climate models work? ;-)

Yes, they all give different numbers. It just shows how complex
environmental physics really is.

 

[Nicholas Kinar]

Phil was teasing. He does that sometimes.

I think that I read too much into this. Hmm... "adding what it ought to
look like"

;-)

 

[Glen Walpert]

Crickey, what is it made of?

Those Vishay Bulk Metal resistors:

http://www.vishay.com/resistors-discrete/metal-foil/

are made of a proprietary variant of 75Ni-20Cr-3Al, which is roughly
an order of magnitude better than Shunt Manganin, and is the lowest
resistance tempco material known in the vicinity of room temperature.
One commercially available variant is known as Evanohm IIRC, available
as foil and wire in case you want to make your own resistors .

When comparing tempcos of different materials it is important also to
compare the specified temperature range or better yet look at the
resistance vs temp curve.

Low resistance TC metal alloys get their low TC the same way Invar
gets its low thermal expansion. Invar, at one specific temperature,
exactly cancels thermal expansion with a gradual phase change to a
lower volume phase. Low TC resistance alloys like Manganin and NiCrAl
exactly cancel tempco at one specific temperature with a gradual phase
change to a lower resistance phase. The further you get from the
temperature which is exactly compensated the worse the tempco in
either case. Specify TC very close to the compensated temp and you
can make it arbitrarily low .

Regards,
Glen

 

[Jim Thompson]

Thanks. I appreciate it much better with the explanation of how it
works.

How about OpAmp offset voltage? That'll do you in royally, unless you
have an auto-zero periodically.

...Jim Thompson
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