Page 7 of the application note gives this hint. "With a reference voltage at less than half of VDD, one input can be tied to the reference voltage, and the other input can range from 0V to 2 · VREF. "
How to measure non differential signal with differential ADC?
Feb 24, 2005
9 Replies
Hi - I need to measure a signal that will range from about 0-5 V with a very high accuracy ADC. I was planning on using the TI ADS1252U
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- as it's inexpensive, fast, and accurate. Only one major question though - how do I connect a non differential signal to differential inputs? It seems to me that every single ADC I've looked at has differential inputs. In the datasheet for the ADS1252U they show how to connect a bi-polar signal, but unless I'm getting my terminology confused I don't believe that that is what I want. So - what should I do? Thanks so much for your help!-Michael Noone
This will, however, cost you a bit of resolution, as the ADC is able to swing from an input of +4 to -4 differentially, and grounding the input, while the simpler solution by far, limits you to +4 to 0. Alternatively you can use a good clean voltage reference and either a fully differential amplifier such as the OPA1632 or two traditional op-amps to let the signal use the entire differential range. This will improve linearity somewhat and resolution by one bit, but it'll also cost a couple dollars.
If the signal is referenced to ground and positive, one plan might be to ground the -Vin pin on the ADS1252. The device will measure the voltage difference between the two pins, so don't choose your ground unwisely. The signals have to be within the common mode range of the converter, and I've not checked what that is- sometimes it varies with the mode of the converter (eg. buffered vs. not etc.)
The ADC input range is to +/-Vref, limited by Vin(+) and Vin(-) >
-300mV. If the Vref and Vdd are (say) 5.100V, then the range is -300mV to 5.1V, which covers the OP's 0-5V requirement with some room to spare.
The LSB of a 24-bit ADC seldom means very much (you need to average
4096 readings on this part to have a hope of getting the 24th bit of effective resolution).This part doesn't seem to be spec'd very well for DC accuracy, despite the suggested applications which require it.
There is a lot of good application data in "Techncial Documents" here:
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Best regards, Spehro Pefhany
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:
Ah! I had been worried about losing resolution if I ran it at it's "full speed" - but I had looked through the datasheet and couldn't find anything relating effective resolution to frequency. Now - as soon as you've mentioned it - I see the graph was right in front of me the entire time! Doh...
Ideally I'd like to get 20 bits resolution at 20khz, so I'm starting to think that I should use a different ADC. The other chip I've been eyeing is the TI ADS1271:
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I've looked through it twice now - and I still can't find anything about frequency and resolution. Best I've been able to find is that it has three modes - high resolution, high frequency, and low power - but I can't find any numbers for those. Oh, and if 20 bits at 20khz is not possible - I'd much rather sacrifice resolution than speed. Does the ADS1271 look like a good solution? I see they have a diagram showing a OPA1632 being used to connect a single ended interface input.
What do you think?
Spehro Pefhany wrote in news: snipped-for-privacy@4ax.com:
It was my understanding that with differential inputs the two inputs should be the same magnitude and opposite signs. Is this not correct? It wouldn't cause any problems for the negative input to be grounded?
-Michael
Rob Gaddi wrote in news:cvj8fu $ snipped-for-privacy@gazette.corp.bcm.tmc.edu:
with
to
input,
Alternatively
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Cost is not a serious worry right now. I mean sure - it's always great to be the hero and find a solution using really cheap components - but this will be replacing a multiple thousand dollar system, so a couple dollars won't really break the bank :)
-Michael
How are Linear Technology's No Latency Delta Sigma converters in this regard ? 10 Hz rate is fine for me.
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