AD7891 Просмотр технического описания (PDF) - Analog Devices
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AD7891 Datasheet PDF : 20 Pages
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AD7891
PIN FUNCTION DESCRIPTIONS
PLCC MQFP
Pin No. Pin No. Mnemonic
1–5 28–43
34–44
VINXA, VINXB
10, 19
11, 33
20
6
4, 13
5, 27
14
44
VDD
AGND
DGND
STANDBY
9
3
REF OUT/REF IN
7
1
REF GND
30
24
CONVST
32
26
12
6
EOC
MODE
Description
Analog Input Channels. The AD7891 contains eight pairs of analog input channels. Each
channel contains two input pins to allow a number of different input ranges to be used
with the AD7891. There are two possible input voltage ranges on the AD7891-1. The
± 5 V input range is selected by connecting the input voltage to both VINXA and VINXB,
while the ± 10 V input range is selected by applying the input voltage to VINXA and con-
necting VINXB to AGND. The AD7891-2 has three possible input ranges. The 0 V to
2.5 V input range is selected by connecting the analog input voltage to both VINXA and VINXB; the
0 V to 5 V input range is selected by applying the input voltage to VINXA and connecting
VINXB to AGND while the ± 2.5 V input range is selected by connecting the analog input
voltage to VINXA and connecting VINXB to REF IN (provided this REF IN voltage comes
from a low impedance source). The channel to be converted is selected by the A2, A1,
and A0 bits of the control register. In the parallel interface mode, these bits are available as
three data input lines (DB3 to DB5) in a parallel write operation. While in the serial inter-
face mode, these three bits are accessed via the DATA IN line in a serial write operation.
The multiplexer has guaranteed break-before-make operation.
Positive Supply Voltage, 5 V ± 5%.
Analog Ground. Ground reference for track/hold, comparator, and DAC.
Digital Ground. Ground reference for digital circuitry.
Standby Mode Input. TTL compatible input used to put the device into the power
save or standby mode. The STANDBY input is high for normal operation and low for
standby operation.
Voltage Reference Output/Input. The part can either be used with its own internal refer-
ence or with an external reference source. The on-chip 2.5 V reference voltage is pro-
vided at this pin. When using this internal reference as the reference source for the
part, REF OUT should be decoupled to REF GND with a 0.1 mF disc ceramic capaci-
tor. The output impedance of the reference source is typically 2 kW. When using an
external reference source as the reference voltage for the part, the reference source
should be connected to this pin. This overdrives the internal reference and provides the
reference source for the part. The reference pin is buffered on-chip but must be able to
sink or source current through this 2 kW resistor to the output of the on-chip reference.
The nominal reference voltage for correct operation of the AD7891 is 2.5 V.
Reference Ground. Ground reference for the part’s on-chip reference buffer. The REF
OUT pin of the part should be decoupled with a 0.1 mF capacitor to this REF GND
pin. If the AD7891 is used with an external reference, the external reference should also
be decoupled to this pin. The REF GND pin should be connected to the AGND pin
or the system’s AGND plane.
Convert Start. Edge-triggered logic input. A low-to-high transition on this input puts
the track/hold into hold and initiates conversion. When changing channels on the part,
sufficient time should be given for multiplexer settling and track/hold acquisition between
the channel change and the rising edge of CONVST.
End-of-Conversion. Active low logic output indicating converter status. The end of con-
version is signified by a low-going pulse on this line. The duration of this EOC pulse is
nominally 80 ns.
Interface Mode. Control input that determines the interface mode for the part. With this
pin at a logic low, the AD7891 is in its serial interface mode; with this pin at a logic high,
the device is in its parallel interface mode.
–6–
REV. D
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