LTC2984 - control of ADC1, ADC2 and ADC3 configuration...

Just pinging this question again... hoping to gain more understand of the capabilities and limitations of this chip.

Hi Jonathan,

Can you share the block diagram of the application you have in mind? Are you planning to do a direct ADC measurement or an RTD measurement?

Regards,

Mae

I would like to do something that looks like this...

Where i can measure the resistance of two units simultaneously while using a controlled current through the sense resistor.  I do not need a calculated temperature output, but a simultaneous voltage or resistance reading would be wonderful.

How do I go about gaining direct access to ADC3 to force it to measure something at the same time as ADC1 and ADC2?

You may refer to page 61 in the datasheet or to this thread for voltage measurement topic. Let me know if this answers your question. 

For me to better understand your needs, may I know why you need a secondary measurement? 

Thank you for the link to the thread.  It still does not help to answer my question.  This IC has three ADCs and they can all take measurements simultaneously.  I want to take advantage of that fact... how do I go about doing it?  In what normal case does the IC use all three ADCs simultaneously?

Why?  There are two reasons that quickly come to mind... First, redundant measurements of the same device by two ADCs simultaneously can provide more noise immunity.  Second, simultaneous measurement of the sense resistor and two different components can provide twice the throughput in a large matrix measurement system.

On page 13 the datasheet indicates "The LTC2984 automatically generates the excitation current, simultaneously measures the sense resistor and thermistor/RTD voltage..."  This appears to use only two of the ADCs.  How do I activate a measurement scheme in which I can simultaneously measure using all 3 ADCs.

Is there anyone that can assist with this request?  I am happy to discuss offline via email if needed.

Hi JCheney ,

Unfortunately, there is no way to configure the three channels of the ADC so that you can use them all simultaneously in your desired application. The ADCs are configured depending on the type of sensor you're trying to measure. If you're trying to measure using a resistive element, 2 ADCs will simultaneously sample, one for the temperature sensor and another for the sense resistor. If however you're trying to measure using a thermocouple and you're using a resistive element to determine the temperature of the cold junction, the 3 ADCs will sample simultaneously. One for the voltage induced by the thermocouple, another for the temperature sensor for the cold junction compensation, and the last for the sense resistor. 

I hope I was able to answer your question. 

Regards,
Karlo

Karlo,

Thank you for the response.  "So you are saying there is a chance."  I guess I could just configure the unit to read a "thermocouple" and then I will be able to sample all three ADCs simultaneously.  In this configuration, are all three measurements external to the IC or is the cold junction measurement internal (not using external pins)?

Again, thank you for the feedback.

Best regards,

Jonathan

Hi Jonathan, 

All the three measurements are external to the IC. However, I don't think you'll be able to sample 3 different temperature sensors simultaneously as the part just wasn't designed to do that. For example, if you configure the unit to read a thermocouple and you use a thermistor or RTD for the cold junction compensation, one of the ADCs will be dedicated to measuring Rsense, which means it will affect the other two measurements (i.e. thermocouple, and resistor element channels).

Regards,
Karlo