ADS1118 common mode range - Page 1
Forum section: electronics / projects

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 06:17:39 am
content: This is the chiphttps://www.ti.com/lit/ds/symlink/ads1118.pdfI am trying to work out whether the input Vcm includes GND. The data sheet implies it does, but how can it? The input mux can handle GND - most MOSFET switches (e.g. 4016) can. The ADC is a switched-cap design and they usually can. But the PGA? How can you make the output of an op-amp go all the way to GND? Obviously you cannot.Someone herehttps://electronics.stackexchange.com/questions/443212/can-the-ads1118-adc-converter-accurately-read-a-voltage-close-to-gndasked the same question but missed the key part about the PGA.Experimentallyit does seem to work, however. I am testing it down to -150mV or so and it is converting correctly. I am using it mostly with the bottom-right switch closed i.e. AIN0-3 are single ended inputs relative to the GND pin.So I am wondering whether they hacked the PGA to deliver a differential output which is slightly shifted up, and then the ADC's differential input can handle that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 20, 2023, 06:59:09 am
content: I mean, if it's working, that's empirical confirmation that your hypothesis is "obviously" false.Keep in mind, you're never going to get confirmation from the manufacturer about how it works, at least short of averymotivating curiosity with very in-depth NDAs locking things down, or becoming an employee at TI and happening to land in the department responsible for the design of this device or friends.Also keep in mind that TI's block diagrams are low information. They are definitely not to be taken at face value, and rarely are internally consistent. So they might show a "PGA" block here, using an amplifier symbol, and that's functionally what seems to be present, but the internal structure can be anything at all that exhibits equivalent behavior.As for how they might -- it could simply be an instrumentation amp with built-in input offset, intentionally tuned to avoid having to output GND, and then compensated later in the signal path. It might be a different scheme (than the standard three-opamp in-amp). They could even include an onboard charge pump to create a minor rail extension, completely avoiding the problem with the conventional circuit.It might not even be a voltage amplifier at all. Since the ADC is switched-capacitor, why not the PGA? An input could be sampled, level-shifted to standard common mode range, and boosted (perhaps several times, regeneratively, i.e. sampling the output and feeding it back to the input), and the final value S&H'd to the D-S core. (A sampled method might show a lot more noise, though.)Someone more knowledgeable about ADC design might be able to chime in; it's not hard to come up with lots of possibilities; knowing which ones are likely, will require some familiarity with the craft.Tim

author: magic
author_rank: Super Contributor
timestamp: August 20, 2023, 08:00:13 am
content: Quote

author: Eraldo
author_rank: Regular Contributor
timestamp: August 20, 2023, 08:15:30 am
content: As always reading the datasheet in detail almost always gives you the answers to your questions. Their sole purpose is to give detailed info on the product you are implementing in your design.So if you think you read all of it, read it again because you definetly missed some things that will either help in your design process or help you learn new things.(I too am doing my research on the ads127L11 adc and i learned a lot since i had no experience with adc's. )

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 08:34:19 am
content: QuoteThat was what I was thinking. Could they be varying Vref instead, to achieve the "PGA"?I suspect that might be it, IOW the ADC is connected directly to the input pins. The supporting evidence is in9.3.2 Analog Inputswhere they state that the input impedance of the ADC is effectively resistive, but they would not need to say anything if the PGA was anything normal made up of op-amps or similar.So, yeah, the PGA block is bogusThat also explains why the gain ratio matching is super accurate - in the ppm range. The Vref is within about 0.1% (which itself is very good) but the .256 / .512 etc gains are really well matched.

author: dietert1
author_rank: Super Contributor
timestamp: August 20, 2023, 08:44:00 am
content: The input common mode range has little to do with output rail to rail capability. Most likely the PGA is of the instrumentation amplifier type and they configure gain by switching resistors. The statement about the sampling capacitor is about the built-in ADC.Regards, Dieter

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 08:48:45 am
content: Not the way I read it. If the PGA was some active circuit, they would not need to even discuss the ADC input characteristics. And the output of the PGA could not possibly swing below GND.See my edited post (it crossed). I think there is no BGA as such; the mux feeds the ADC switched-cap input directly. They are just switching Vref.

author: dietert1
author_rank: Super Contributor
timestamp: August 20, 2023, 09:09:28 am
content: There is nothing like bogus in a TI datasheet. If their block diagram shows a PGA, the device includes a PGA.Regards, Dieter

author: magic
author_rank: Super Contributor
timestamp: August 20, 2023, 09:15:17 am
content: Quote from: dietert1 on August 20, 2023, 08:44:00 amDoubt it, because they say it's connected to the input pins, and the first thing in the chip is the PGA, not the ADC.All those precision ΔΣ ADCs with PGAs likely use tricks like in LTC1043 datasheet (read it), both for a rail-to-rail input stage with high CMRR and for accurate gain.Or maybe some other tricks with capacitors. But that sampling capacitor frontend is suspiciously common.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 10:28:52 am
content: OK, let me rephrase it. The PGA isn't a normal three op-amp one. It is some equivalent function. But then why discuss the ADC input characteristics?They could have put in a Vcm spec at least. It would have been really handy.

author: magic
author_rank: Super Contributor
timestamp: August 20, 2023, 10:43:15 am
content: It would be nice, I didn't see anything like that in the datasheet.However, they do say that differential input range is limited only by supply voltage and PGA gain, which seems to imply that the inputs should be usable rail to rail.edit"V(AINx)absolute input voltage" is specified as GND to VCC in "recommended operating conditions". I think this is the line which covers it, if there is any.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 10:53:05 am
content: They say you should not turn on the input protection diodes, which implies you can go down to about -0.3V (the typical diode Vf when the leakage is still negligible) and my tests suggest that works.

author: dietert1
author_rank: Super Contributor
timestamp: August 20, 2023, 11:26:42 am
content: If they realize the PGA with switched capacitors instead of resistors, one needs buffer caps on the inputs, like shown in Figure 50 in the ADS1118 datasheet. No problem at max. 800 samples per second.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2023, 12:08:56 pm
content: One way to realize the gain with a switched capacitor SD ADC is to increase the rate of input sampling. So there is never an amplified voltage, just the capacitor charged more often.This way they get the rather low TC and quite accurate gain (though no I have not found the specs for the ADS1118).A hint on that some kind of capacitor switching is used comes from the change in the input impedance with the gain - though it is rather high for the ADS1118.

author: peter-h
author_rank: Super Contributor
timestamp: August 20, 2023, 02:09:36 pm
content: That makes sense.The data sheet explicitly states that the "capacitor switching" is seen at the input pins.

author: peter-h
author_rank: Super Contributor
timestamp: August 21, 2023, 11:30:28 am
content: FWIW this is the sort of accuracy I am seeing on my test procedureTLV2333 driving the ADS1118.5 microvolt zero error.Gain errors worst case 330ppm (0.033%). This is way better than the internal Vref spec.It's an amazing device.

author: iMo
author_rank: Super Contributor
timestamp: August 21, 2023, 11:57:10 am
content: I've been using the ADS1110 6pin chips and yes, nice chips. Can measure a high side shunt voltage as well.BTW it comes from Burr-Brown, afaik..

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2023, 12:23:18 pm
content: I am guessing the ADS1118 uses charge pumps internally to generate gate voltages for the FET's in the sampler and multiplexer switches. and those pumped up voltages will already be (probably several volts) outside of both the GND and positive supply rails. Circuitry around this may be similar to a level converter for (for example) driving a 24V relay coil with a 5V microcontroller.And with those charge pumps. the first thing that happens if the input voltage gets outside of the supply rails is that the ESD diodes start conducting.

author: peter-h
author_rank: Super Contributor
timestamp: August 21, 2023, 01:22:37 pm
content: I don't think you need the extra rails. You can make analog switches which are rail to rail, e.g. MAX4510, and that's all you need to make "switched capacitor anything" AIUI.Below GND, and probably above VCC, operation is then automatic.The relay coil is ok too but I get your drift. This FET is 3.3V driven.Burr Brown made good stuff.ADS1110 looks nice too but is I2C; I much prefer SPI.

author: Warhawk
author_rank: Frequent Contributor
timestamp: August 21, 2023, 01:55:55 pm
content: If you want an ultimate answer then I suggest you going to E2E forum (on ti.com) and asking there. People responsible for that part get the notification and have to answer it. I know that extra registration is annoying. However, you get often a real answer unlike wild speculations on what is inside (which is what happened here).

author: peter-h
author_rank: Super Contributor
timestamp: August 21, 2023, 02:56:45 pm
content: Not possible -Error 403You don't have permission to access this page. If you think this is a mistake, try these steps...https://e2e.ti.com/search?q=&category=forumseems to be the only way to reach the registration.No TI ID number, no contact options at all AFAICS.

author: dietert1
author_rank: Super Contributor
timestamp: August 21, 2023, 03:20:27 pm
content: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1084131/ads1118-nonlinearity-around-0?tisearch=e2e-sitesearch&keymatch=ads1118#This appears to be a relevant answer i found without registration. In total 402 answers about ADS1118.Regards, Dieter

author: peter-h
author_rank: Super Contributor
timestamp: August 21, 2023, 03:42:55 pm
content: It looks like the device has some "bias current" issues around zero, and that user found it because he is feeding the device via resistors of a significant value.That result is not all that surprising. Well, TI should have documented this, since their appnote shows resistors in series with the inputs...Luckily, I am feeding the device directly from TLV2333 outputs. I did that for several reasons, one of which is that the DS does show Zin to be pretty low on most of the ranges and frankly these values (which come with no assurance of being constant) ought to ring alarm bells with any series resistorsBut that post, together with TI's lack of comment on the input going below GND, confirms that inputs < GNDaresupposed to work. As indeed I have found, except that I have not done precise measurements.

author: peter-h
author_rank: Super Contributor
timestamp: August 22, 2023, 10:23:09 am
content: The ADS1120 specifies it betterBetter chip; should have used it instead.But that uses a classic PGA made up of chopper-stabilised op-amps. But you can bypass it ("Disabling the PGA lowers the overall power consumption andalso removes the restrictions of Equation 13 through Equation 15 for the common-mode input voltage range,VCM. The usable absolute and common-mode input voltage range is (AVSS – 0.1 V ≤ V(AINx), VCM ≤ AVDD +0.1 V) when the PGA is disabled.") andonly thencan Vcm go below GND.