Microchip Technology MA330028 Data Sheet

DS70000657H-page 358

 2011-2013 Microchip Technology Inc.

There are two configurations to take into consider-
ation when designing with the op amp modules that
are available in the dsPIC33EPXXXGP50X,
dsPIC33EPXXXMC20X/50X and PIC24EPXXXGP/
MC20X devices. Configuration A (see 

)

takes advantage of the internal connection to the ADC
module to route the output of the op amp directly to the
ADC for measurement. Configuration B (see

) requires that the designer externally route

the output of the op amp (OAxOUT) to a separate ana-
log input pin (ANy) on the device. 

 describes

the performance characteristics for the op amps, distin-
guishing between the two configuration types where
applicable.

25.1.1

OP AMP CONFIGURATION A

 shows a typical inverting amplifier circuit

taking advantage of the internal connections from the
op amp output to the input of the ADC. The advantage of
this configuration is that the user does not need to con-
sume another analog input (ANy) on the device, and
allows the user to simultaneously sample all three op
amps with the ADC module, if needed. However, the
presence of the internal resistance, R

INT

1, adds an error

in the feedback path. Since R

INT

1 is an internal resis-

tance, in relation to the op amp output (V

OA

x

OUT

) and

ADC internal connection (V

ADC

), R

INT

1 must be included

in the numerator term of the transfer function. See

 in 

 for the typical value of R

INT

1. 

 and

 in 

 describe the minimum sample time (T

SAMP

)

requirements for the ADC module in this configuration.

 also defines the equations that should be

used when calculating the expected voltages at points,
V

ADC

 and V

OAXOUT

.

–

+

CxIN1-

CxIN1+

R

1

ADC

OAxOUT

R

INT

1

R

FEEDBACK(2)

OAx

(to ADC)

Op Ampx

See 

 for the Typical value.

See 

 for the Minimum value for the feedback resistor.

See 

 and 

 for the minimum sample time (T

SAMP

).

CV

REF1O

 or CV

REF2O

 are two options that are available for supplying bias voltage to the op amps.

V

IN

V

ADC

(V

OA

X

OUT

)

1

------------------------------









–





=

INT1

+

1

---------------------------------------------------









–





=

Bias

Voltage