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# THS4541: THS4541 about the resistance value question

Part Number: THS4541

Hi team,

The customer would like to use DEV-ADC34J22. For the DEV-ADC34J22, it has the THS4541 circuit.

On page 8 of the DEV-ADC34J22 schematic, R35 is 59 ohm, R28 is 205 and R27 is 27.4ohm.

The customer would like to know how to calculate the R35, R28 and R27 values?

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

• Hi Mickey,

Do you know what gain configuration is the customer planning to use the THS4541 at? The detailed design procedure to calculate the resistor values for a particular gain setting is provided in Section 9.4.1.3 of the THS4541 datasheet.

Best Regards,
Rohit
• In reply to Rohit Bhat:

Hi Rohit,

The User' Guide shows the GAIN for THS4541 circuit is 2V/V. But I check the Table 1 of THS4541, when gain is set 2,
the resistance value is not equal to R35 59 ohm, R28 205ohm and R27 27.4ohm.

The customer now cannot understand this, and he does not start to design the circuit.

• In reply to Mickey Zhang:

Hi Mickey,

The user's guide shows the gain of the channel to be 2 V/V and not for the THS4541 alone. I believe the THS4541 gain is set to 2.35V/V in-order to compensate for the 0.86V/V loss between the THS4541 and the ADC due to the voltage division across R27, R30 and R31. As a result, the overall gain for the entire channel is 2.02V/V.

The other aspect to note is that the R35, R27 and R28 values will change if the Rf value is different for the same gain (say 2V/V). The user's guide has an Rf of 499 ohms, whereas the datasheet table 1 calculates the different resistor values based on Rf of 402 ohms.

If the customer is looking to design the THS4541 at gain of 2, then the datasheet table values should hold good for an Rf of 402-ohms. Please let us know if there are further questions or there is help needed in determining the resistor values for a particular gain setting.

Best Regards,
Rohit