ADUC847BCP8-5 AD [Analog Devices], ADUC847BCP8-5 Datasheet - Page 32

• Manufacturer Part Number: ADUC847BCP8-5
• Description: MicroConverter Multichannel 24-/16-Bit ADCs with Embedded 62 kB Flash and Single-Cycle MCU
• Manufacturer: AD [Analog Devices]

ADuC845/ADuC847/ADuC848
AUXILIARY ADC (ADUC845 ONLY)
Table 18. ADuC845 Typical Output RMS Noise (µV) vs.
Update Rate with Chop Enabled
SF Word
13
23
27
69
255
Table 19. ADuC845 Typical Peak-to-Peak Resolution (Bits) vs.
Update Rate
SF Word
13
23
27
69
255
1
Table 20. ADuC845 Typical Output RMS Noise (µV) vs.
Update Rate with Chop Disabled
SF Word
3
13
66
69
81
255
Table 21. ADuC845 Peak-to-Peak Resolution (Bits) vs.
Update Rate with Chop Disabled
SF Word
3
13
66
69
81
255
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 AV
2.5 V, with the primary and auxiliary (ADuC845 only) reference
select bits configured from the ADC0CON2 and ADC1CON
(ADuC845 only), respectively.
ADC converting in bipolar mode.
Data Update Rate (Hz)
1365.33
315.08
62.06
59.36
50.57
16.06
1
Data Update Rate (Hz)
1365.33
315.08
62.06
59.36
50.57
16.06
with Chop Enabled
Data Update Rate (Hz)
105.03
59.36
50.56
19.79
5.35
105.03
59.36
50.56
19.79
5.35
Data Update Rate (Hz)
DD
. The nominal external reference voltage is
µV
17.46
3.13
4.56
2.66
1.13
Bits
15.5
18
17.5
18
19.5
µV
1386.58
34.94
3.2
3.19
3.14
1.71
Bits
9
14.5
18
18
18
19
Rev. A | Page 32 of 108
When an external reference voltage is used, the primary ADC
sees this internally as a 2.56 V reference (V
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
(2.56/2
or ADuC847). If a bipolar gain of 4 is used (±640 mV), the LSB
size is (±640 mV)/2
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 µF). 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 200 µA constant
current generators, one sourcing current from the AV
AIN(+), and one sinking current from AIN(−) to AGND. These
currents are only configurable for use on AIN4 to AIN5 and/or
AIN6 to AIN7 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 of the input voltage on the analog input channel
24
) = 152.6 nV (if using the 24-bit ADC on the ADuC845
24
) = 76.3 nV (again using the 24-bit ADC
REF
× 1.024).
DD
to