ADS1256 reference details - Page 1
Forum section: electronics / projects

author: dietert1
author_rank: Super Contributor
timestamp: September 05, 2020, 06:49:29 pm
content: When looking at the circuitry of an Arroyo TECPAK 585 temperature controller i noticed something strange. Although the ADS1256 datasheet has a clear statement that a reference with low output impedance is needed to realize the precision that ADC offers, they included a filter with two 49R9 resistors in front of REFN and REFP, so a source impedance of 100 Ohm for the reference voltage. Each of those two resistors shows 5.8 mV, while with a 2.5 V reference and the nominal 18K5 input resistance of the ADS1256 one would expect 6.5 mV.Now each mV represents 400 ppm of a 2.5 V reference, so there is an enormous potential for errors, for example if the input impedance of the ADS1256 has the high TC typical of semiconductors. Maybe it's a design error and there should be inductors instead of resistors.But today i found yet another schematic with the same filter in it. Anybody knows the reason of those 49R9 resistors?Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2020, 08:44:22 pm
content: The ADC wants quite some capacitance at the reference inputs. I am not so sure just a differential capacitance is enough. The normal OPs have a problem driving such a large capacitance directly. So the 50 Ohms may be just the isolation of the amplifiers output, skipping the full capacitance tolerant circuit.The 18.5 K input impedance is a nominal values - so the 5.8 mV are not that far off the expected values. The inputs are ear certain sampling with some switch and internal capacitor. So there may very well not be much true resistance involved. Chip internal capacitors can be quite good and stable.Am external inductor may also help, but it's a 2 sided thing. Real inductors also have a DC resistance and this usually has a high TC. To compete with a 25 ppm/K resistor, the inductor would need to be less than 1/150 the resistance, thus < 0.3 Ohms in the example.

author: The Soulman
author_rank: Super Contributor
timestamp: September 05, 2020, 09:22:45 pm
content: The current flowing into the v-ref-in isn't a steady current, its pulsed.The group of capacitors provide a low impedance source for those short current spikes.

author: dietert1
author_rank: Super Contributor
timestamp: September 05, 2020, 10:03:22 pm
content: In a little hair dryer test the voltage across the 49R9 resistor decreased from 5.93 to 5.89 mV. Now 40 uV are 16 ppm of 2.5 V. Two of those resistors give 32 ppm. So this method makes the ADS1256 a 15 bit ADC. One reason to find the same strange circuit twice would be that one designer copied it from the other. Now the Arroyo temperature controller is from 2010, while the waveshare/seed board was introduced in 2008, so i suspect they used one of those boards during development of their temperature controllers and later kept the same circuit as "good enough".Just shorted out both resistors and the TECPAK continues to work very well, except now the calibration of the thermistor resistance measurement is off by -4.9E-3, more or less as expected (-100/18600). Reference voltage looks quiet on the scope.TI recommends using a REF6225 buffered reference that drives 47uF without problem and with about 0.125 Ohm output impedance. It is available with the same 3 ppm/K like the ADR441B used in the Arroyo TECPAK, except it has a factor two more voltage noise. By the way, the REF62xx datasheet is a very interesting read for anybody trying to build a reference meant to drive an ADC or a precision meter without dips. Those references support 4-wire output so it may even be possible to eliminate the copper TC of the signal tracks.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 06, 2020, 07:37:09 am
content: The TC of the resistors would add to the overall gain TC. The ADS1256 is specified with typical 0.8 ppm/K.The resistors have an effect of about R/18K, so the TC of an 100 Ohms resistor would be attenuated by about a factor if 180. So one should not have the very cheapest resistors, but the requirement on the resistor drift would not be that high.The more troublesome point is that the input current from the reference may depend on the signal - that would cause INL errors. I have not checked the ADS1256 data-sheet or real HW, but this may happen.

author: dietert1
author_rank: Super Contributor
timestamp: September 07, 2020, 09:01:49 am
content: Thanks for the reminder. Checking the ADS1256 datasheet i found the proposal of 49R9 resistors in Fig. 25. Then there is a footnote that mentions the reference buffer in Fig. 26. For me it is a bit confusing. On page 17 they also mention the temperature coefficient of the reference input current to be approximately 35ppm/°C. Times 2*49,9/18500 gives 0,11 ppm/K (negligible). Need to study this further.On my.TI they recommend using a REF6225, it was released after the ADS1256 (and its datasheet).https://e2e.ti.com/support/data-converters/f/73/p/884178/3270008?tisearch=e2e-quicksearch&keymatch=ref6225#3270008Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2022, 07:52:04 am
content: Recently i found a mistake on one of those chinese ADS1256 boards offered on amazon and ebay.The circuit is very similar to the one proposed in the ADS1256 datasheet, so it includes an OPA350 reference buffer, with a low pass filter between the reference and the buffer input. The datasheet proposal is 10K + 1 uF. The designer put two MLCC capacitors there. I measured 7.5 uF (probably 10 uF + 100 nF nominal).In a simple leakage current test those MLCCs had 1000x more leakage than the 1 uF film capacitor that is meant to be there. Voltage drop over the 10 KOhm is about 20 uV when cold. That's about 8 ppm of 2.5 V, with a large temperature effect. Input leakage of the OPA350 is at most 10 pA, about a factor 200 less.Conclusion: replace those MLCCs by a film capacitor.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2022, 08:10:08 am
content: X7R and similar capacitors have quite a lot of dielectric absorbtion and this includes slow parts. This makes it difficult to measure the true leakage and separate it from DA related current. Another point with capacitors is that they are also temperature dependent. A changing temperature also leads to a current flow in of out of the capacitor. A film capacitor would be the better choice.The OPA350 amplifier is anyway somewhat limiting to the precision. It is a nice, fast CMOS amplifier, but the DC / low frequency performance is limited. So the circuit would not provide best DC performance anyway.

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2022, 10:58:28 am
content: If you want to propose an upgrade part, you are welcome.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2022, 12:02:42 pm
content: Driving the reference is tricky, as one has a quite large capacitance at the output and still needs a fast driver. Chances are one would need a kind of compound driver. The OPA350, which seems to be OK with the capacitance, could be used for the fast part and than an 2nd OP (e.g. OP07 or similar) for low frequency correction.

author: iMo
author_rank: Super Contributor
timestamp: September 02, 2022, 02:39:56 pm
content: Would be great to see a schematics, with the compound driver powered by 5V and outputting say 2.5 and 4.096V (as the sigma-delta adcs usually use).

author: dietert1
author_rank: Super Contributor
timestamp: September 02, 2022, 08:31:31 pm
content: The OPA350 has no noise spec in the 0.1 to 10 Hz range. If we look at the OPA340 datasheet, the schematic is similar and it exhibits a very similar noise spectrum, with a spec of 8 uVrms from 100 Hz up, so even more in the 0.1 to 10 Hz range. A better amplifier is the OPA140, with 42 nVrms in 0.1 to 10 Hz. And it works from > 4.5 V. For stability it may need three more parts though, two resistors and a small cap. The circuit is here:https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg3269318/#msg3269318.As far as i understand, the datasheet circuit already works in two separate frequency bands. While high frequencies are fed to the ADC by the buffer capacitors, lower frequencies should come from the buffer amplifier. Maybe a chopper amplifier with low input current could also be used.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: September 15, 2022, 01:59:31 pm
content: At 5 V supply the OPA140 can't be used to buffer the 2.5 V reference, as it requires a headroom of 3.5 V above the inputs.Better choices were OPA335 or MAX4238. Those require a headroom of 1.5 V or so and work very well in this application, of course with the extra two passives (one resistor, one capcitor). The MAX4238 requires a connection on nSHDN (Pin1).These cheap ADS1256 boards come with a pretty good ADR03B reference (TC better than +/- 1 ppm/K).Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2022, 04:00:50 pm
content: The reference drive must be tolerant to quite some capacitance at the output and should be resonable fast, so that the referemce values recovers reasonable fast. This is not at all an easy task.

author: dietert1
author_rank: Super Contributor
timestamp: September 15, 2022, 06:35:24 pm
content: Sounds like you have a question.In those cheap ADS1256 boards i am using there are three capacitors buffering the reference input of the ADS1256, total capacitance about 60 uF. The Keithley circuit i linked above is driving that capacitance with stability. I checked that down to about 50 uV. Of course the 1 uF/10 KOhm low pass filter between reference and buffer needs shielding, otherwise it will pick up 50 Hz mains.One may think about the ESR of those buffer capacitors. Every Ohm can create voltage errors of about 500 uV peak to peak, but nothing was observed at the scale of 50 uV.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2022, 07:30:59 am
content: The ESR of the buffer capacitors can be imporant. If driven directly from the amplifer, it is not about getting the lowest possible ESR. Some ESR in the combination can be required to keep it stable and at least help dampening ringing. Often it helps to have some capacitance with very low ESR and some capacitance with some ESR for damping.

author: dietert1
author_rank: Super Contributor
timestamp: September 16, 2022, 09:02:48 am
content: Capacitor impedance as a function of frequency usually has a V shape, with the minimum at a characteristic frequency and at some milliOhms in the case of MLCCs or foil caps. That's why people combine several caps of different shape and size. Thus the V minimum should become a shallow minimum, i.e. a low impedance over an extended frequency range.Another design trap is the crossover frequency between buffering by the caps (high pass) and the buffer amplifier (low pass). In the design i am using the high pass extends down to about 50 Hz and the amplifier works up to about 100 KHz. With the 300 Ohms and 1 nF shown above, it could even be 1 MHz. So there is a large overlap to suppress residual reference disturbances by about 60 dB. And you start at 20 mV (average voltage over amplifier output resistor), so the result is once more stability down to about 20 uV.Regards, Dieter