EVAL-ADUC836QS AD [Analog Devices], EVAL-ADUC836QS Datasheet - Page 26

• Manufacturer Part Number: EVAL-ADUC836QS
• Description: MicroConverter, Dual 16-Bit ADCs with Embedded 62 kB Flash MCU
• Manufacturer: AD [Analog Devices]

Reference Input
The ADuC836’s reference inputs, REFIN(+) and REFIN(–),
provide a differential reference input capability. The common-
mode range for these differential inputs is from AGND to AV
The nominal reference voltage, V
for specifi ed operation is 2.5 V with the primary and auxiliary
reference enable bits set in the respective ADC0CON and/or
ADC1CON SFRs.
The part is also functional (although not specifi ed for per for -
mance) when the XREF0 or XREF1 bits are 0, which enables the
on-chip internal band gap reference. In this mode, the ADCs will see
the internal reference of 1.25 V, therefore halving all input ranges.
As a result of using the internal reference voltage, a noticeable
degradation in peak-to-peak resolution will result. Therefore, for
best performance, operation with an external reference is strongly
rec om mend ed.
In applications where the excitation (voltage or current) for the
transducer on the analog input also drives the reference voltage
for the part, the effect of the low frequency noise in the ex ci ta tion
source will be removed as the application is ratiometric. If the
ADuC836 is not used in a ratiometric application, a low noise
reference should be used. Rec om mend ed reference volt age sources
for the ADuC836 include the AD780, REF43, and REF192.
It should also be noted that the reference inputs provide a high
impedance, dynamic load. Because the input impedance of each
reference input is dynamic, resistor/capacitor combinations on these
inputs can cause dc gain errors depending on the output imped-
ance of the source that is driving the reference inputs. Reference
voltage sources, like those recommended above (e.g., AD780),
will typically have low output impedances and there fore decoupling
capacitors on the REFIN(+) input would be recommended.
Deriving the reference input voltage across an external resistor,
as shown in Figure 66, will mean that the reference input sees a
signifi cant external source impedance. External decoupling on
the REFIN(+) and REFIN(–) pins would not be recommended
in this type of circuit confi guration.
Burnout Currents
The primary ADC on the ADuC836 contains two 100 nA con-
stant current generators: one sourcing current from AV
AIN(+) and one sinking from AIN(–) to AGND. The currents
are switched to the selected analog input pair. Both currents are
either on or off, depending on the Burnout Current Enable
(BO) bit in the ICON SFR (see Table IX). These currents can
be used to verify that an external transducer is still operational
before attempting to take measurements on that channel. Once
the burnout currents are turned on, they will fl ow in the ex ter nal
transducer circuit, and a measurement of the input voltage on
the analog input channel can be taken. If the resultant volt age
measured is full-scale, it indicates that the transducer has gone
open-circuit. If the voltage measured is 0 V, it indicates that the
transducer has short circuited. For normal operation, these burn-
out currents are turned off by writing a 0 to the BO bit in the
ICON SFR. The current sources work over the nor mal absolute
input voltage range specifi cations.
ADuC836
REF
(REFIN(+) – REFIN(–)),
DD
to
DD
.
–26–
Excitation Currents
The ADuC836 also contains two identical, 200 µA constant current
sources. Both source current from AV
(IEXC2). These current sources are con trolled via bits in the ICON
SFR shown in Table IX. They can be confi gured to source 200 µA
individually to both pins or a combination of both currents, i.e.,
400 µA, to either of the selected pins. These current sources can be
used to excite ex ter nal resistive bridge or RTD sensors.
Reference Detect
The ADuC836 includes on-chip circuitry to detect if the part has
a valid reference for conversions or calibrations. If the volt age
between the external REFIN(+) and REFIN(–) pins goes below
0.3 V or either the REFIN(+) or REFIN(–) inputs is open circuit,
the ADuC836 detects that it no longer has a valid reference. In this
case, the NOXREF bit of the ADCSTAT SFR is set to a 1. If the
ADuC836 is performing normal con ver sions and the NOXREF
bit becomes active, the con ver sion results revert to all 1s. It is not
necessary to continuously mon i tor the status of the NOXREF bit
when performing con ver sions. It is only necessary to verify its status
if the con ver sion result read from the ADC Data Register is all 1s.
If the ADuC836 is performing either an offset or gain cal i bra tion
and the NOXREF bit becomes active, the updating of the respec-
tive calibration registers is inhibited to avoid loading incorrect
coeffi cients to these registers, and the appropriate ERR0 or ERR1
bits in the ADCSTAT SFR are set. If the user is concerned about
verifying that a valid reference is in place every time a calibration is
performed, the status of the ERR0 or ERR1 bit should be checked
at the end of the calibration cycle.
A - ADC generally consists of two main blocks, an analog modu-
lator and a digital fi lter. In the case of the ADuC836 ADCs, the
analog modulators consist of a difference amplifi er, an integrator
block, a comparator, and a feedback DAC, as illustrated in Figure 10.
In operation, the analog signal sample is fed to the difference
amplifi er along with the output of the feedback DAC. The dif-
ference between these two signals is integrated and fed to the
comparator. The output of the comparator provides the input to
the feedback DAC so the system functions as a negative feed back
loop that tries to minimize the difference signal. The dig i tal data
that represents the analog input voltage is contained in the duty
cycle of the pulse train appearing at the output of the comparator.
This duty cycle data can be recovered as a data-word using a sub-
sequent digital fi lter stage. The sampling fre quen cy of the modulator
loop is many times higher than the bandwidth of the input signal.
The integrator in the modulator shapes the quantization noise
(which results from the analog-to-digital conversion) so that the
noise is pushed toward one-half of the modulator frequency.
ANALOG
- Modulator
INPUT
Figure 10.
DIFFERENCE
AMP
-
Modulator Simplifi ed Block Di a gram
INTEGRATOR
DAC
DD
COMPARATOR
to Pin 3 (IEXC1) or Pin 4
HIGH
FREQUENCY
BIT STREAM
TO DIGITAL
FILTER
REV. 0