ADUC834 Analog Devices, ADUC834 Datasheet - Page 26

• Manufacturer Part Number: ADUC834
• Description: Precision Analog Microcontroller: 1MIPS 8052 MCU + 62kB Flash + 16/24-Bit ADC + 12-Bit DAC
• Manufacturer: Analog Devices

Specifications of ADUC834

• Mcu Core: 8052
• Mcu Speed (mips): 1
• Sram (bytes): 2304Bytes
• Gpio Pins: 34
• Adc # Channels: 4
• Other: PWM

ADuC834
Reference Input
The ADuC834’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 specified 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 specified for performance)
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 recommended.
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 excitation
source will be removed as the application is ratiometric. If the
ADuC834 is not used in a ratiometric application, a low noise
reference should be used. Recommended reference voltage sources
for the ADuC834 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
impedance 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
therefore 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 refer-
ence input sees a significant external source impedance. External
decoupling on the REFIN(+) and REFIN(–) pins would not be
recommended in this type of circuit configuration.
Burnout Currents
The primary ADC on the ADuC834 contains two 100 nA constant
current generators, one sourcing current from AV
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 cur-
rents are turned on, they will flow in the external transducer
circuit, and a measurement of the input voltage on the analog
input channel can be taken. If the resultant voltage measured is
full-scale, it indicates that the transducer has gone open-circuit.
REF
(REFIN(+) – REFIN(–)),
DD
to AIN(+),
DD
.
–26–
If the voltage measured is 0 V, it indicates that the transducer
has short circuited. For normal operation, these burnout currents
are turned off by writing a 0 to the BO bit in the ICON SFR.
The current sources work over the normal absolute input voltage
range specifications.
Excitation Currents
The ADuC834 also contains two identical, 200 A constant
current sources. Both source current from AV
or Pin 4 (IEXC2). These current sources are controlled via bits
in the ICON SFR shown in Table IX. They can be configured
to source 200 A individually to both pins or a combination of
both currents, i.e., 400 A, to either of the selected pins. These
current sources can be used to excite external resistive bridge or
RTD sensors.
Reference Detect
The ADuC834 includes on-chip circuitry to detect if the part has
a valid reference for conversions or calibrations. If the voltage
between the external REFIN(+) and REFIN(–) pins goes below
0.3 V or either the REFIN(+) or REFIN(–) inputs is open circuit,
the ADuC834 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 ADuC834 is performing normal conversions and the
NOXREF bit becomes active, the conversion results revert to all
1s. It is not necessary to continuously monitor the status of the
NOXREF bit when performing conversions. It is only necessary
to verify its status if the conversion result read from the ADC
Data Register is all 1s.
If the ADuC834 is performing either an offset or gain calibration
and the NOXREF bit becomes active, the updating of the respec-
tive calibration registers is inhibited to avoid loading incorrect
coefficients 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
modulator and a digital filter. In the case of the ADuC834
ADCs, the analog modulators consist of a difference amplifier,
an integrator block, a comparator, and a feedback DAC as
illustrated in Figure 10.
ANALOG
- Modulator
INPUT
Figure 10.
DIFFERENCE
AMP
-
Modulator Simplified Block Diagram
INTEGRATOR
DAC
COMPARATOR
DD
to Pin 3 (IEXC1)
HIGH
FREQUENCY
BITSTREAM
TO DIGITAL
FILTER
REV. A