ADUC816 Analog Devices, ADUC816 Datasheet - Page 34

• Manufacturer Part Number: ADUC816
• Description: Precision Analog Microcontroller: 1MIPS 8052 MCU + 8kB Flash + Dual 16-Bit ADC + 12-Bit DAC
• Manufacturer: Analog Devices

Specifications of ADUC816

• Mcu Core: 8052
• Mcu Speed (mips): 1.3
• Sram (bytes): 256Bytes
• Gpio Pins: 34
• Adc # Channels: 4

The absolute input voltage range on the auxiliary ADC is restricted
to between AGND – 30 mV to AVDD + 30 mV. The slightly
negative absolute input voltage limit does allow the possibility of
monitoring small signal bipolar signals using the single-ended
auxiliary ADC front end.
Programmable Gain Amplifier
The output from the buffer on the primary ADC is applied to the
input of the on-chip programmable gain amplifier (PGA). The
PGA can be programmed through eight different unipolar input
ranges and bipolar ranges. The PGA gain range is programmed
via the range bits in the ADC0CON SFR. With the external refer-
ence select bit set in the ADC0CON SFR and an external 2.5 V
reference, the unipolar ranges are 0 mV to +20 mV, 0 mV to
40 mV, 0 mV to 80 mV, 0 mV to 160 mV, 0 mV to 320 mV,
0 mV to 640 mV and 0 V to 1.28 V and 0 to 2.56 V while the
bipolar ranges are ± 20 mV, ± 40 mV, ± 80 mV, ± 160 mV,
±320 mV, ±640 mV, ±1.28 V and ±2.56 V. These are the nominal
ranges that should appear at the input to the on-chip PGA. An
ADC range matching specification of 0.5 LSB (typ) across all
ranges means that calibration need only be carried out at a single
gain range and does not have to be repeated when the PGA
gain range is changed.
The auxiliary ADC does not incorporate a PGA and is configured
for a fixed single input range of 0 to V
Bipolar/Unipolar Inputs
The analog inputs on the ADuC816 can accept either uni-
polar or bipolar input voltage ranges. Bipolar input ranges
do not imply that the part can handle negative voltages with
respect to system AGND.
Unipolar and bipolar signals on the AIN(+) input on the primary
ADC are referenced to the voltage on the respective AIN(–) input.
For example, if AIN(–) is 2.5 V and the primary ADC is config-
ured for an analog input range of 0 mV to +20 mV, the input
voltage range on the AIN(+) input is 2.5 V to 2.52 V. If AIN(–)
is 2.5 V and the ADuC816 is configured for an analog input range
of 1.28 V, the analog input range on the AIN(+) input is 1.22 V
to 3.78 V (i.e., 2.5 V ± 1.28 V).
As mentioned earlier, the auxiliary ADC input is a single-ended
input with respect to the system AGND. In this context a bipolar
signal on the auxiliary ADC can only span 30 mV negative
with respect to AGND before violating the voltage input limits
for this ADC.
Bipolar or unipolar options are chosen by programming the
Primary and Auxiliary Unipolar enable bits in the ADC0CON
and ADC1CON SFRs respectively. This programs the relevant
ADC for either unipolar or bipolar operation. Programming for
either unipolar or bipolar operation does not change any of the
input signal conditioning; it simply changes the data output coding
and the points on the transfer function where calibrations occur.
When an ADC is configured for unipolar operation, the output
coding is natural (straight) binary with a zero differential input
voltage resulting in a code of 000 . . . 000, a midscale voltage
resulting in a code of 100 . . . 000, and a full-scale input voltage
resulting in a code of 111 . . . 111. When an ADC is configured
for bipolar operation, the coding is offset binary with a negative
full-scale voltage resulting in a code of 000 . . . 000, a zero
differential voltage resulting in a code of 100 . . . 000, and a
positive full-scale voltage resulting in a code of 111 . . . 111.
REF
.
Burnout Currents
The primary ADC on the ADuC816 contains two 100 nA con-
stant current generators, one sourcing current from AVDD to
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 VIII). 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 flow in the exter-
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, this 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
burnout currents are turned off by writing a 0 to the BO bit in
the ICON SFR. The current sources work over the normal abso-
lute input voltage range specifications.
Excitation Currents
The ADuC816 also contains two identical, 200 μA constant
current sources. Both source current from AVDD to Pin 3
(IEXC1) or Pin 4 (IEXC2) These current sources are con-
trolled via bits in the ICON SFR shown in Table VIII. They
can be configured 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 exter-
nal resistive bridge or RTD sensors.
Reference Input
The ADuC816’s reference inputs, REFIN(+) and REFIN(–),
provide a differential reference input capability. The common-
mode range for these differential inputs is from AGND to AVDD.
The nominal reference voltage, VREF (REFIN(+) – REFIN(–)),
for specified operation is 2.5 V with the primary and auxil-
iary reference enable bits set in the respective ADC0CON
and/or ADC1CON SFRs.
The part is also functional (although not specified for perfor-
mance) when the XREF0 or XREF1 bits are “0,” which enables
the on-chip internal bandgap 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 volt-
age, a noticeable degradation in peak-to-peak resolution will
result. Therefore, for best performance, operation with an exter-
nal 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 excita-
tion source will be removed as the application is ratiometric. If the
ADuC816 is not used in a ratiometric application, a low noise
reference should be used. Recommended reference voltage sources
for the ADuC816 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 recom-
REV. A