ADUC824 Analog Devices, ADUC824 Datasheet - Page 34

• Manufacturer Part Number: ADUC824
• Description: Precision Analog Microcontroller: 1MIPS 8052 MCU + 8kB Flash + 16/24-Bit ADC + 12-Bit DAC
• Manufacturer: Analog Devices

Specifications of ADUC824

• Mcu Core: 8052
• Mcu Speed (mips): 1
• Sram (bytes): 256Bytes
• Gpio Pins: 34
• Adc # Channels: 4

ADuC824
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, 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 2 µV (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.
Typical matching across ranges is shown in Figure 20 below.
Here, the primary ADC is configured in bipolar mode with an
external 2.5 V reference, while just greater than 19 mV is forced
on its inputs. The ADC continuously converts the DC input
voltage at an update rate of 5.35 Hz, i.e., SF = FFhex. In total,
800 conversion results are gathered. The first 100 results are
gathered with the primary ADC operating in the ± 20 mV range.
The ADC range is then switched to ± 40 mV and 100 more con-
version results are gathered, and so on until the last group of
100 samples are gathered with the ADC configured in the ± 2.56 V
range. From Figure 20, The variation in the sample mean
through each range, i.e., the range matching, is seen to be of
the order of 2 µV.
The auxiliary ADC does not incorporate a PGA and is configured
for a fixed single input range of 0 to V
SAMPLE COUNT
ADC RANGE
19.372
19.371
19.370
19.369
19.368
19.367
19.366
19.365
19.364
0
100
200
300
400
REF
.
500
600
700
800
Bipolar/Unipolar Inputs
The analog inputs on the ADuC824 can accept either unipolar 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 ADuC824 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.
Burnout Currents
The primary ADC on the ADuC824 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.