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ADuC845/ADuC847/ADuC848
Data Sheet
Rev. C | Page 32 of 108
AUXILIARY ADC (ADUC845 ONLY)
Table 18. ADuC845 Typical Output RMS Noise (V) vs.
Update Rate with Chop Enabled
SF Word
Data Update Rate (Hz)
V
13
105.03
17.46
23
59.36
3.13
27
50.56
4.56
69
19.79
2.66
255
5.35
1.13
Table 19. ADuC845 Typical Peak-to-Peak Resolution (Bits) vs.
Update Rate1 with Chop Enabled SF Word
Data Update Rate (Hz)
Bits
13
105.03
15.5
23
59.36
18
27
50.56
17.5
69
19.79
18
255
5.35
19.5
1
ADC converting in bipolar mode.
Table 20. ADuC845 Typical Output RMS Noise (V) vs.
Update Rate with Chop Disabled
SF Word
Data Update Rate (Hz)
V
3
1365.33
1386.58
13
315.08
34.94
66
62.06
3.2
69
59.36
3.19
81
50.57
3.14
255
16.06
1.71
Table 21. ADuC845 Peak-to-Peak Resolution (Bits) vs.
Update Rate with Chop Disabled
SF Word
Data Update Rate (Hz)
Bits
3
1365.33
9
13
315.08
14.5
66
62.06
18
69
59.36
18
81
50.57
18
255
16.06
19
REFERENCE INPUTS
The ADuC845/ADuC847/ADuC848 each have two separate
differential reference inputs, REFIN± and REFIN2±. While
both references are available for use with the primary ADC,
only REFIN± is available for the auxiliary ADC (ADuC845 only).
The common-mode range for these differential references is
from AGND to AVDD. The nominal external reference voltage is
2.5 V, with the primary and auxiliary (ADuC845 only) reference
select bits configured from the ADC0CON2 and ADC1CON
(ADuC845 only), respectively.
When an external reference voltage is used, the primary ADC
sees this internally as a 2.56 V reference (VREF × 1.024).
Therefore, any calculations of LSB size should account for this.
For instance, with a 2.5 V external reference connected and
using a gain of 1 on a unipolar range (2.56 V), the LSB size is
(2.56/224) = 152.6 nV (if using the 24-bit ADC on the ADuC845
or ADuC847). If a bipolar gain of 4 is used (±640 mV), the LSB
size is (±640 mV)/224) = 76.3 nV (again using the 24-bit ADC
on the ADuC845 or ADuC847).
The ADuC845/ADuC847/ADuC848 can also be configured to
use the on-chip band gap reference via the XREF0/1 bits in the
ADC0CON2 SFR (for primary ADC) or the AXREF bit in
ADC1CON (for auxiliary ADC (ADuC845 only)). In this mode
of operation, the ADC sees the internal reference of 1.25 V,
thereby halving all the input ranges. A consequence of using the
internal band gap reference is a noticeable degradation in peak-
to-peak resolution. For this reason, operation with an external
reference is recommended.
In applications where the excitation (voltage or current) for the
transducer on the analog input also drives the reference inputs
for the part, the effect of any low frequency noise in the
excitation source is removed because the application is ratio-
metric. If the parts are not used in a ratiometric configuration, a
low noise reference should be used. Recommended reference
voltage sources for the ADuC845/ADuC847/ADuC848 include
ADR421, REF43, and REF192.
The reference inputs provide a high impedance, dynamic load
to external connections. Because the impedance of each reference
input is dynamic, resistor/capacitor combinations on these pins
can cause dc gain errors, depending on the output impedance of
the source that is driving the reference inputs. Reference voltage
sources, such as those mentioned above, for example, the ADR421,
typically have low output impedances, and, therefore, decoupling
capacitors on the REFIN± or REFIN2± inputs would be recom-
mended (typically 0.1 F). Deriving the reference voltage from
an external resistor configuration means that the reference input
sees a significant external source impedance. External decoupling
of the REFIN± and/or REFIN2± inputs is not recommended in
this type of configuration.
BURNOUT CURRENT SOURCES
The primary ADC on the ADuC845 and the ADC on the
ADuC847 and ADuC848 incorporate two 100 nA constant
current generators that are used to detect a failure in a connected
sensor. One sources current from the AVDD to AIN(+), and one
sinks current from AIN() to AGND. These currents are only
configurable for use on AIN5/AIN6 and/or AIN7/AIN8 in
differential mode only, from the ICON.6 bit in the ICON SFR
(see
Table 30). These burnout current sources are also available
only with buffering enabled via the BUF0/BUF1 bits in the
ADC0CON1 SFR. Once the burnout currents are turned on, a
current flows in the external transducer circuit, and a measurement