Datasheet

ADuC836
–24–
ADuC836
–25–
The auxiliary ADC has three external input pins (labeled AIN3
to AIN5) as well as an internal connection to the on-chip tem-
perature sensor. All inputs to the auxiliary ADC are single-ended
inputs referenced to the AGND on the part. Channel selection
bits in the ADC1CON SFR detailed in Table VII allow selection
of one of four inputs.
Two input multiplexers switch the selected input channel to the
on-chip buffer amplier in the case of the primary ADC and
directly to the - modulator input in the case of the auxiliary
ADC. When the analog input channel is switched, the settling
time of the part must elapse before a new valid word is available
from the ADC.
MU
X
AIN3
AIN4
AIN5
ON-CHI
P
TEMPERATURE
SENSO
R
-
MODULATOR
PROGRAMMABL
E
DIGITAL FILTE
R
- ADC
REFIN(–
)
REFIN(+
)
CHOP
OUTPUT
AVERAG
E
OUTPUT
SCALIN
G
DIGTAL OUTPUT
RESULT WRITTE
N
TO ADC1H/L SFRs
CHOP
- ADC
THE - ARCHITECTURE
ENSURES 16 BITS NO MISSING
CODES. THE ENTIRE
- ADC
IS CHOPPED TO REMOVE
DRIFT ERRORS
.
MUX
OUTPUT AVERAGE
AS PART OF THE CHOPPING
IMPLEMENTATION, EACH
DATA-WORD OUTPUT
FROM THE FILTER IS
SUMMED AND AVERAGED
WITH ITS PREDECESSO
R
TO NULL ADC CHANNEL
OFFSET ERRORS.
THE MODULATOR PROVIDES A
HIGH FREQUENCY 1-BIT DATA
STREAM (THE OUTPUT OF WHICH
IS ALSO CHOPPED) TO TH
E
DIGITAL FILTER,
THE DUTY CYCLE OF WHICH
REPRESENTS THE SAMPLED
ANALOG INPUT VOLTAGE.
- MODULATOR
PROGRAMMABLE DIGITAL
FILTE
R
THE SINC
3
FILTER REMOVES
QUANTIZATION NOISE INTRODUCE
D
BY THE MODULATOR. THE UPDAT
E
RATE AND BANDWIDTH OF THIS
FILTER ARE PROGRAMMABLE
VIA THE SF SFR.
THE OUPUT WORD FROM THE
DIGITAL FILTER IS SCALED BY
THE CALIBRATIO
N
COEFFICIENTS BEFORE
BEING PROVIDED AS
THE CONVERSION RESULT.
OUTPUT SCALING
ANALOG INPUT CHOPPING
THE INPUTS ARE ALTERNATELY
REVERSED THROUGH THE
CONVERSION CYCLE. CHOPPING
YIELDS EXCELLENT ADC
OFFSET AND OFFSET DRIFT
PERFORMANCE
.
DIFFERENTIAL REFERENCE
THE EXTERNAL REFERENCE INPUT
TO THE ADuC836 IS DIFFERENTIAL
AND FACILITATES RATIOMETRIC
OPERATION. THE EXTERNAL
REFERENCE VOLTAGE IS SELECTE
D
VIA THE XREF1 BIT IN ADC1CON.
REFERENCE DETECT
CIRCUITRY TESTS FOR OPEN OR
SHORTED REFERENCE INPUTS.
ANALOG MULTIPLEXER
A DIFFERENTIAL MULTIPLEXER
ALLOWS SELECTION OF THRE
E
EXTERNAL SINGLE ENDED INPUT
S
OR THE ON-CHIP TEMP. SENSOR.
THE MULTIPLEXER IS CONTROLLE
D
VIA THE CHANNEL SELECTION
BITS IN ADC1CON.
Figure 8. Auxiliary ADC Block Diagram
Auxiliary ADC
The auxiliary ADC is intended to convert supplementary inputs
such as those from a cold junction diode or thermistor. This ADC
is not buffered and has a xed input range of 0 V to 2.5 V (assuming
an external 2.5 V reference). The single-ended inputs can be driven
from AIN3, AIN4, or AIN5 pins, or directly from the on-chip
temperature sensor voltage. A block diagram of the auxiliary ADC
is shown in Figure 8.
Analog Input Channels
The primary ADC has four associated analog input pins (labeled
AIN1 to AIN4) that can be congured as two fully differential input
channels. Channel selection bits in the ADC0CON SFR detailed
in Table VI allow three combinations of differential pair selection
as well as an additional shorted input option (AIN2–AIN2).
REV. A
REV. A