aduc847bs8-3 Analog Devices, Inc., aduc847bs8-3 Datasheet - Page 22

• Manufacturer Part Number: aduc847bs8-3
• Description: Microconverter, 10-channel 24-bit Adc With Embedded 62kb Flash Mcu
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
TABLE VI: Typical Output rms noise (µV) vs Input Range and Update Rate for the ADuC847 with chopping disabled.
SF
Word
TABLE VII: Peak to Peak Resolution (bits) vs Input Range and Update Rate for the ADuC847 with chopping disabled.
SF
Word
Reference Inputs
The ADuC847 has two separate differential reference inputs
REFIN+/- and REFIN2+/-. The common mode range for these
differential references is from AGND to AV
external reference voltage is 2.5v, with the reference select bits
configured from the ADC0CON2 and respectively.
The ADuC847 can also be configured to use the on-chip band-gap
reference, via the XREF0/1 bits in the ADC0CON2 SFR. In this
mode of operation the ADCs will see the internal reference of
1.25v, thereby halving all the input ranges. A consequence of using
the internal bandgap reference is a noticable degradation in peak-
to-peak resolution. For this reason 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 inputs for
the part, the effect of the of any low frequency noise in the
excitation source will be removed as the application is retiometric.
If the ADuC847 is not used in a ratiometric configuration then a
low noise reference should be used. Recommended references
voltage sources for the ADuC847 include ADR421, REF43,
REF192.
It should also be noted that 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 referenc
inputs. Reference voltage sources, like those mentioned above (e.g.
ADR421), will typically have low output impedances and therefore
REV. PrA 05/03
255
255
69
69
3
3
Data Update
Rate (Hz)
Data Update
Rate (Hz)
1365.0
1365.0
59.36
16.06
59.36
16.06
±20 mV
±20 mV
0.961
0.475
2.47
11
13
14
DD
±40 mV
±40 mV
. The nominal
Figure: 7 ADC Circuit with CHOP disabled
0.971
0.468
2.49
12
14
15
±80 mV
±80 mV
0.949
0.434
2.37
14
15
16
22
±160 mV
±160 mV
0.922
0.485
3.87
14
16
17
decoupling capacitors of the REFIN+/- or REFIN2+/- inputs would
be recommended. Deriving the reference voltage from an external
resistor configuration will mean that the reference input sees a
significant external source impedance. External decoupling of the
REFIN+/- and/or REFIN2+/- inputs would not be recommended in
this type of configuration.
Burnout Current Sources
The ADC on the ADuC847 incorporates two 200uA constant
current generators, one sourcing current from the AV
and one sinking current from AIN(-) to AGND. These currents are
only configurable for use on AIN4 → AIN5 and/or AIN6 → AIN7
in differential mode only, from the BO bit in the ICON SFR.
These burnout current sources are also only available with full
buffering enabled via the BUF0/BUF1 bits in the ADC0CON1
SFR. Once the burnout currents are turned on, a current will flow in
the external transducer circuit, and a measurement of the input
voltage on the analog input channel can be taken. If the resulting
voltage measured is fullscale, it indicates that the transducer has
gone open-circuit. If the voltage measured is 0v, it indicates that
the transducer has gone short circuit. The current sources work over
the normal absolute input voltage range specifications.
Reference Detect Circuit
The ADC has the option of using the internal bandgap reference or
an external reference applied to the two REFIN pins, by means of
the XREF0/1 bits in the control registers AD0CON2. A reference
detection circuit is provided to detect whether there is a valid
voltage applied to the REFIN+/- pins. This feature arose in
connection with strain gauge sensors in weigh-scales where the
reference and signal are provided via a cable from the remote
sensor. It is desirable to detect whether the cable is disconnected.
Input Range
Input Range
±320 mV
±320 mV
0.923
0.458
7.18
14
17
18
±640 mV
±640 mV
12.61
0.688
1.32
14
17
18
±1.28 V
±1.28 V
16.65
ADuC847
2.03
1.18
15
18
19
±2.56 V
±2.56 V
32.45
3.73
1.78
15
18
19
DD
to AIN(+),