Other Parts Discussed in Thread: TMS320F28335, TMS320F2812
Hi
As mentioned in Footnote 6, page 168, datasheet tms320F28335.pdf, the ADCREFP and ADCREFM are sourced by a single reference, that can be internal or external. This is accounted by the fact, that the two signals are tracking together. In my opinion, this makes sense, if ADCREFP and ADCREFM can be sourced by two external references that have the same common mode error. This option is provided on the F2812. But are ADCREFP and ADCREFM also egligible on the F28338?
example TMS320F2812
Vr1: reference voltage 1
Vr2: reference voltage 2
ecm : common mode error
Gd : Gain of difference amplifier
Vrefp = Vr1 + ecm
Vrefm = Vr2 + ecm
Vrefadc = (Vrefp - Vrefm) * Gd
--->Vrefadc = ( Vr1 - Vr2 ) * Gd
The common mode error is eliminated.
example TMS320F28335
Vs : single reference voltage
es : single reference error
Gp : Gain for positive reference
Gm : Gain for medium reference
Vrefp' = ( Vs + es ) * Gp
Vrefm' = ( Vs + es ) * Gm
Vrefadc' = ( Vrefp' - Vrefm' ) * Gd
---> Vrefadc' = Vs * [ Gd ( Gp - Gm ) ] + es * [ Gd ( Gp - Gm ) ]
The error term is amplified by the same term as the reference voltage....
Maybe there is an other effect I didn't think about? Thank you for your help.
Kind Regards!
Timmy