Datasheet

ADuC832 Data Sheet
Rev. B | Page 40 of 92
Table 20. Some Single-Supply Op Amps
Op Amp Model Characteristics
OP281/OP481 Micropower
OP191/OP291/OP491 I/O Good up to V
DD
, low cost
OP196/OP296/OP496 I/O to V
DD
, micropower, low cost
OP183/OP249 High gain-bandwidth product (GBP)
OP162/OP262/OP462 High GBP, micro package
AD820/AD822/AD824 FET input, low cost
AD823 FET input, high GBP
Keep in mind that the ADC’s transfer function is 0 V to V
REF
,
and any signal range lost to amplifier saturation near ground
impacts dynamic range. Though the op amps in Table 20 are
capable of delivering output signals very closely approaching
ground, no amplifier can deliver signals all the way to ground
when powered by a single supply. Therefore, if a negative supply
is available, consider using it to power the front-end amplifiers.
However, be sure to include the Schottky diodes shown in
Figure 40 (or at least the lower of the two diodes) to protect the
analog input from undervoltage conditions. In summary, use the
circuit of Figure 40 to drive the analog input ADCx pins of the
ADuC832.
VOLTAGE REFERENCE CONNECTIONS
The on-chip 2.5 V band gap voltage reference can be used as
the reference source for the ADC and DACs. To ensure the
accuracy of the voltage reference, the user must decouple the
V
REF
pin to ground with a 0.1 µF capacitor, and the C
REF
pin to
ground with a 0.1 µF capacitor, as shown in Figure 41.
BUFFER
BUFFER
0.1µF
0.1µF
51Ω
V
REF
C
REF
2.5V
BAND GAP
REFERENCE
ADuC832
02987-030
Figure 41. Decoupling V
REF
and C
REF
If the internal voltage reference is to be used as a reference for
external circuitry, the C
REF
output should be used. However, a
buffer must be used in this case to ensure that no current is
drawn from the C
REF
pin itself. The voltage on the C
REF
pin is
that of an internal node within the buffer block, and its voltage
is critical to ADC and DAC accuracy. On the ADuC812, V
REF
is
the recommended output for the external reference; this can be
used but note that there is a gain error between this reference
and that of the ADC.
The ADuC832 powers up with its internal voltage reference in
the on state. This is available at the V
REF
pin, but as noted
previously, there is a gain error between this and that of the
ADC. The C
REF
output becomes available when the ADC is
powered up.
If an external voltage reference is preferred, it should be con-
nected to the V
REF
and C
REF
pins as shown in Figure 42. Bit 6 of
the ADCCON1 SFR must be set to 1 to switch in the external
reference voltage.
To ensure accurate ADC operation, the voltage applied to V
REF
must be between 1 V and AV
DD
. In situations where analog
input signals are proportional to the power supply (such as
some strain gage applications), it may be desirable to connect
the C
REF
and V
REF
pins directly to AV
DD
.
Operation of the ADC or DACs with a reference voltage below
1 V, however, may incur loss of accuracy, eventually resulting in
missing codes or nonmonotonicity. For that reason, do not use
a reference voltage less than 1 V.
0.1µF
0.1µF
51Ω
BUFFER
V
REF
C
REF
EXTERNAL
VOLTAGE
REFERENCE
1 = EXTERNAL
0 = INTERNAL
2.5V
BAND GAP
REFERENCE
ADuC832
ADCCON1[6]
V
DD
02987-031
Figure 42. Using an External Voltage Reference
To maintain compatibility with the ADuC812, the external
reference can also be connected to the V
REF
pin, as shown in
Figure 43, to overdrive the internal reference. Note that this
introduces a gain error for the ADC that has to be calibrated
out; thus the previous method is the recommended one for
most users. For this method to work, ADCCON1[6] should be
configured to use the internal reference. The external reference
then overdrives this.