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TLA2021: Front End for TLA2021

Claire Rauchfuss
Prodigy 50 points
Part Number: TLA2021
Other Parts Discussed in Thread: PGA281, PGA280, ADS1120, ADS1146, ADS114S08

Below is a customer question:

 

"I would appreciate a suggestion for how to put a front end on TLA2021 ADC that would allow me to configure the same input wiring terminal of my product for +/-10V or +/-20mA.

 

Also for this non-isolated product, the analog input reference will be shared with the power supply return.  Any suggestions on how best to keep this noise immune will also be of interest to me."

 

Any advice would be appreciated. Please let me know if/what additional information you need to help the customer.

  • 0
    TI__Guru* 97825 points
    Claire,


    I don't know of any good universal front end solution for you that doesn't require some changes from ±10V to ±20mA inputs. The problem is that for a ±20mA input, you would want a small resistive load to sink the current for measurement so that the resulting voltage isn't overly large. At the same time, during a ±10V measurement you would probably want a large resistive input to act as a voltage divider so that the ±10V source isn't required to source large loads. I've seen cases where customers will build separate inputs and switch one or the other in for measurement.

    Another aspect of your question is: how do you take a bipolar input (±10V or ±20mA) and make it single ended (to within 0 to 5V or 0 to 3.3V)? In that case, you would probably need to buffer the resistive load for either the voltage input or current input, and then use some sort of difference amplifier, centering the reference input to mid scale of the supply and have the input vary around that reference voltage. Instead of a difference amplifier, you could use a PGA280 or PGA281 as well (that would also require ±15V supplies).

    One other way to make this bipolar measurement is use a data converter that has bipolar analog supplies. Devices like the ADS1146, ADS1120, and the ADS114S08 all have supplies that support ±2.5V analog supplies, while the digital supply can be run at +3.3V. This way, you could use a voltage divider for the voltage measurement and a simple resistive current sink for the current measurement. It would still require a change of inputs from one to the other measurement, but it would be the simplest front end.


    Joseph Wu