AD7865
AD7865-3
Figure 4 shows the analog input section of the AD7865-3. The
analog input range is
±
2.5 V on the V
INxA
input. The V
INxB
input can be left unconnected but if it is connected to a poten-
tial then that potential must be AGND.
+2.5V
REFERENCE
6k
V
REF
R1
R2
V
INxA
V
INxB
TRACK/
HOLD
TO INTERNAL
COMPARATOR
TO ADC
REFERENCE
CIRCUITRY
AD7865-3
Figure 5 shows the arrangement used. The
H/S
SEL controls a
multiplexer that selects the source of the conversion sequence
information, i.e., from the hardware channel select pins (SL1 to
SL4) or from the channel selection register. When a conversion
is started the output from the multiplexer is latched until the
end-of-the conversion sequence. The data bus bits DB0 to DB3
(DB0 representing Channel 1 through DB3 representing Chan-
nel 4) are bidirectional and become inputs to the channel select
register when
RD
is logic high and
CS
and
WR
are logic low.
The logic state on DB0 to DB3 is latched into the channel select
register when
WR
goes logic high. Figure 6 shows the loading
sequence for channel selection using software control. When
using software control to select the conversion sequence a write
is only required each time the conversion sequence needs chang-
ing. This is because the channel select register will hold its in-
formation until different information is written to it.
It should be noted that the hardware select Pins SL1 and SL2
are dual function. When
H/S
SEL is logic low (selecting the
conversion sequence using software control) they take the func-
tions CLK IN and
INT/EXT
CLK respectively. Therefore, the
logic inputs on these pins must be set according to the type of
operation required (see Using an External Clock). Also when
H/S
SEL is high, the SL3 and SL4 logic inputs have no function
and can be tied either high or low, but should not be left floating.
H/S
HARDWARE CHANNEL
SELECT PINS
SL1
SL2
SL3
SL4
CHANNEL
SELECT
REGISTER
WR
CS
WR
TRANSPARENT WHILE WAITING FOR
CONVST.
LATCHED ON THE RISING EDGE OF
CONVST
AND
DURING A CONVERSION SEQUENCE.
MULTIPLEXER
LATCH
SELECT INDIVIDUAL
TRACK-AND-HOLDS
FOR CONVERSION
Figure 4. AD7865-3 Analog Input Structure
For the AD7865-3, R1 = 4 kΩ and R2 = 4 kΩ. As a result, the
V
INxA
input should be driven from a low impedance source. The
resistor input stage is followed by the high input impedance
stage of the track/hold amplifier.
The designed code transitions take place midway between suc-
cessive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs
etc.) LSB size is given by the formula, 1 LSB = FSR/16384.
Output coding is twos complement binary with 1 LSB = FSR/
16384 = 5 V/16384 = 610.4
µV.
The ideal input/output transfer
function for the AD7865-3 is shown in Table III.
Table III. Ideal Input/Output Code Table for the AD7865-3
DATA BUS
D3 D2 D1 D0
Analog Input
1
+FSR/2 – 3/2 LSB
+FSR/2 – 5/2 LSB
+FSR/2 – 7/2 LSB
AGND + 3/2 LSB
AGND + 1/2 LSB
AGND – 1/2 LSB
AGND – 3/2 LSB
–FSR/2 + 5/2 LSB
–FSR/2 + 3/2 LSB
–FSR/2 + 1/2 LSB
2
Digital Output Code Transition
011 . . . 110 to 011 . . . 111
011 . . . 101 to 011 . . . 110
011 . . . 100 to 011 . . . 101
000 . . . 001 to 000 . . . 010
000 . . . 000 to 000 . . . 001
111 . . . 111 to 000 . . . 000
111 . . . 110 to 111 . . . 111
100 . . . 010 to 100 . . . 011
100 . . . 001 to 100 . . . 010
100 . . . 000 to 100 . . . 001
SEQUENCER
Figure 5. Channel Select Inputs and Registers
RD
t
13
WR
NOTES
1
FSR is full-scale range is 5 V, with V
REF
= +2.5 V.
2
1 LSB = FSR/16384 = 610.4
µV
(± 2.5 V—AD7865-3) with V
REF
= +2.5 V.
t
14
t
15
CS
SELECTING A CONVERSION SEQUENCE
t
16
DATA
t
17
DATA IN
Any subset of the four channels V
IN1
to V
IN4
can be selected for
conversion. The selected channels are converted in an ascending
order. For example if the channel selection includes V
IN4
, V
IN1
and V
IN3
then the conversion sequence will be V
IN1
, V
IN3
and
then V
IN4
. The conversion sequence selection may be made by
using either the hardware channel select input pins SL1 through
SL4 (if
H/S
is tied low) or programming the channel select
register (if
H/S
is tied high). A logic high on a hardware channel
select pin (or logic one in the channel select register) when
CONVST
goes logic high, marks the associated analog input
channel for inclusion in the conversion sequence.
Figure 6. Channel Selection via Software Control
REV. A
–11–
AD [ ANALOG DEVICES ]
