Single-ended/differential input front-end & differential ADC drivers.

[David_]

Single-ended/differential input front-end & differential ADC drivers.

Hello.

I have finally started on a long anticipated project, I want to design measurement tool to be used as a multi-channel volt/ammeter based around a 16-bit ADC. I have some different ideas for this system but I would like to have a circuit for a IV measurement, I have read a lot about ADCs but I have just come up on a problem.

One question to begin with, reading about different ADC drivers it sometimes says "for driving high performance differential succesive approximation register (SAR) ADCs", is it a big difference between driving SAR or delta-sigma ADCs?
Or can I just as well use the quoted part above for a delta-sigma ADC?

I intend to use a ADS1174 Quad, Simultaneous Sampling, 16-Bit ADC.
And that presents me with a couple of obstacles to overcome, one would be the sequencing of the power supplies but more to the point is the front-end. ADS1174 has differential inputs, I have a thought to make two channels voltmeters and the other two ammeters.

But when it comes to the gain of the front-end, is there any obvious choice between a voltage divider in front of the buffer/driver or should one use the driver to scale down the signal?

Using the driver gain to set the attenuation/gain seems difficult, but the ADC will in the end receive 0-3V and I want to be able to measure greater voltages than 3V but I also want to be able to very precise measure very small voltages...
And I think that ether I make a pair of I/V channels, one for high values and one for low values, or two identical setts capable of choosing signal levels.

If anyone have any thoughts about this please advice, I have some more things to ask but lets start wit this.

Regards

- - - Updated - - -

It might add that the ADC will take readings and ether display the values on a 128x64 LCD or send the values to a PC to be processed with Matlab(for some tasks), those two are not one or the other. I will make both available.

 

[gag2000]

Hi,

Looks like good project !

I am using at the moment an ADS8568, wich is a SAR probably suited for your project...
SAR and sigma-delta (SD) are completely different : SAR use a track and hold circuit to "hold" the voltage and the convert it. Hence, an ac voltage can be seen as a successive DC value. They are very precise for DC or low freq AC.
SD use a track and hold too, but convert according to the preceding value, making them suitable for RF or AC signals.

Anyway, a good input stage wiil be valuable in both case : it provides high resistance and low capacitance from the input view, and high speed driving capability ob the output. In can also include some kind of protection for your ADC (diodes ?). And of course, as you stated, to provide a variable gain to use full ADC dynamic range.

To select the gain/atenuation, ADS8568 provides vrefn and verfp : you can probably trim them according to the range : this will limit the number of switches and feedback network on your driving stage.

I would design two identical IV converters with gain range setting : your tool will be more versatile.

My 0.01€

 

[David_]

I don't really know anything or have any real experience with data converters, but if we forgot about the specs of specific IC. Is there such a difference in application of SAR/DS ADC in a DC-low frequency circuit to make a difference, only considering technology?

I would have jumped on the ADC you suggested if it was not for the ADS1174 having ±0,0045 LSB INL(MAX) vs ±3 INL of ADS8568.
But ADS8568 datasheet has DNL spec, ADS1174 don't. Do that speak for ADS1174 not being intended for DC application?

I will go with two identical I/V channels, but I am thinking about if I should implement some circuit to enable one IV channel to be connected to a DUT through only two connections. Kind of like a dedicated IV unit, I don't have any direct idea of what I will measure but for some reason I find my self really wanting to be able to conveniently plot IV plots in matlab. That does seems like great fun.

Are you suggesting that I could have multiple voltage references and switch between them?

 

[FvM]

ADS1174 having ±0,0045 LSB INL

Nope. +/- 0.3 LSB, which is still excellent. An INL specification includes an upper limit for DNL, by the way.

SAR and SD converters have both their specific pros and cons. Obviously SD-ADC have superseded other topologies in the low and medium speed range, you are taking the dataconversion "main road" if you utilize it. Nevertheless you can ruin SD ADC performance by a bad circuit layout, e.g. if you allow clock crosstalk to the input stage.

Regarding the previously asked driver design question, it's preferred to design the differntial ADC driver with a gain around 1, implementing an (anti-aliasing) low-pass filter and the single-ended to differential conversion if required for your application. Voltage dividers or preamplifiers can be put in front.