TMS320F28027: Is the resistance of the AD pin muxed with comparator input different from the other AD pin's?

Aki Li

Part Number: TMS320F28027

Customer found the resistance of A2/B2 and A4/B4 are different from other AD pins' according to the direct measurement at the standalone device. Does this make sense? If so, are there any suggestions to design the AD sampling buffer circuits for these AD pins? Thank you.

over 3 years ago

0 tlee over 3 years ago

TI__Guru 62975 points

Aki,

The ADC Input Model from the datasheet should be used for designing buffer circuits.

Normal resistance measurements are not representative of the pin characteristics because ... the inside of the device is comprised of active circuitry, not passive resistors.

-Tommy

0 Aki Li over 3 years ago in reply to tlee

TI__Expert 8265 points

Tommy,

Can we use the guidance 13.15.2 Choosing an Acquisition Window Duration in F280049 TRM as a reference to configure the S+H time and the 3.4.2.1 Driving ADC Input Pins in SPRAAS1D to design the input buffer? However, if customers don't use amplifier here, how can they judge the R s (ADC driving circuit source impedance)? Thank you.

0 Aki Li over 3 years ago in reply to Aki Li

TI__Expert 8265 points

Tommy,

Could you please help address how can customer judge the Rs (ADC driving circuit source impedance) if they don't use amplifier as the ADC buffer? Thank you.

0 tlee over 3 years ago in reply to Aki Li

TI__Guru 62975 points

Aki,

I recommend using the guidance from the F2806x TRM for ACQPS selection. This information also applies to F2802x and will be incorporated into the F2802x TRM when it is refreshed. It is similar to the F28004x TRM with additional differentiation between high and low bandwidth input signals.

-Tommy

0 Aki Li over 3 years ago in reply to tlee

TI__Expert 8265 points

Tommy,

Thank you for the helpful information.

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TMS320F28027: Is the resistance of the AD pin muxed with comparator input different from the other AD pin's?