Datasheet

REV. B
AD7865
–11–
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.
AD7865-3
V
INxA
TRACK/
HOLD
TO ADC
REFERENCE
CIRCUITRY
TO INTERNAL
COMPARATOR
R1
R2
6k
2.5V
REFERENCE
V
INxB
V
REF
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
Analog Input
1
Digital Output Code Transition
+FSR/2 – 3/2 LSB
2
011 . . . 110 to 011 . . . 111
+FSR/2 – 5/2 LSB 011 . . . 101 to 011 . . . 110
+FSR/2 – 7/2 LSB 011 . . . 100 to 011 . . . 101
AGND + 3/2 LSB 000 . . . 001 to 000 . . . 010
AGND + 1/2 LSB 000 . . . 000 to 000 . . . 001
AGND – 1/2 LSB 111 . . . 111 to 000 . . . 000
AGND – 3/2 LSB 111 . . . 110 to 111 . . . 111
–FSR/2 + 5/2 LSB 100 . . . 010 to 100 . . . 011
–FSR/2 + 3/2 LSB 100 . . . 001 to 100 . . . 010
–FSR/2 + 1/2 LSB 100 . . . 000 to 100 . . . 001
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.
SELECTING A CONVERSION SEQUENCE
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 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
changing. This is because the channel select register will hold its
information 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 high (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.
DATA BUS
D0D1D2D3
WR
CS
WR
CHANNEL
SELECT
REGISTER
SL1
SL2
SL3
SL4
HARDWARE CHANNEL
SELECT PINS
H/S
TRANSPARENT WHILE WAITING FOR CONVST.
LATCHED ON THE RISING EDGE OF CONVST AND
DURING A CONVERSION SEQUENCE.
MULTIPLEXER LATCH
SEQUENCER
SELECT INDIVIDUAL
TRACK-AND-HOLDS
FOR CONVERSION
Figure 5. Channel Select Inputs and Registers
RD
WR
CS
DATA
t
16
t
17
t
14
t
15
DATA IN
t
13
Figure 6. Channel Selection via Software Control