Datasheet
AD7891
–6–
REV. D
PIN FUNCTION DESCRIPTIONS
PLCC MQFP
Pin No. Pin No. Mnemonic Description
1–528–43 V
INXA
, V
INXB
Analog Input Channels. The AD7891 contains eight pairs of analog input channels. Each
34–44 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
± 5V input range is selected by connecting the input voltage to both V
INXA
and V
INXB
,
while the ± 10 V input range is selected by applying the input voltage to V
INXA
and con-
necting V
INXB
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 V
INXA
and V
INXB
; the
0V to 5 V input range is selected by applying the input voltage to V
INXA
and connecting
V
INXB
to AGND while the ± 2.5 V input range is selected by connecting the analog input
voltage to V
INXA
and connecting V
INXB
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.
10, 19 4, 13 V
DD
Positive Supply Voltage, 5 V ± 5%.
11, 33 5, 27 AGND Analog Ground. Ground reference for track/hold, comparator, and DAC.
20 14 DGND Digital Ground. Ground reference for digital circuitry.
644STANDBY 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.
93 REF OUT/REF IN 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.
71 REF GND 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.
30 24 CONVST 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.
32 26 EOC 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.
12 6 MODE 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.