AD7886
BIPOLAR OFFSET AND GAIN ADJUSTMENT
In applications where absolute accuracy is important, offset and
gain error can be adjusted to zero. Offset is adjusted by trim-
ming the voltage at the VIN1 or VIN2 input when the analog in-
put is at zero volts. This can be achieved by adjusting the offset
of an external amplifier used to drive either of these inputs (see
A1 in Figure 9). The trim procedure is as follows:
Apply zero volts at AIN and adjust the offset of A1 until the
ADC output code flickers between 0111 1111 1111 and 1000
0000 0000.
Gain error can be adjusted at either the first code transition
(ADC negative full scale) or the last code transition (ADC posi-
tive full scale). Adjusting the reference, as in Figure 9, will trim
the positive gain error only. The trim procedure is as follows:
Apply a voltage of 4.99756 V, (FS/2–1 LSB) at AIN and
adjust R3 until the output code flickers between 1111 1111
1110 and 1111 11111111.
If the first code transition needs adjusting, a gain trim must be
included in the analog signal path. The trim procedure will then
consist of applying an analog signal of –4.99756 V (–FS/2+1 LSB)
and adjusting the trim until the output code flickers between
0000 0000 0000 and 0000 0000 0001.
+ 5V
AIN
±
5V
Data read operations are controlled by the
CS
and
RD
inputs.
These digital inputs, when low, enable the AD7886’s three-
state output latches. Note, these latches cannot be enabled dur-
ing conversion. In applications where
CS
and
RD
are tied per-
manently low, as in Figure 11, the data bus will go into the
three-state condition at the start of conversion and return to its
active state when conversion is complete. Tying
CS
and
RD
permanently low is useful when external latches are used to
store the conversion results. The data bus becomes active before
BUSY
returns high at the end of conversion, so that
BUSY
can
be used as a clocking signal for the external latches.
A typical DSP application would have a timer connected to the
CONVST
input for precise sampling intervals.
BUSY
would be
connected to the interrupt of a microprocessor that would be
asserted at the end of every conversion. The microprocessor
would then assert the
CS
and
RD
inputs and read the data from
the ADC. For applications where both data reading and conver-
sion control need to be managed by a microprocessor, a
CONVST
pulse can be decoded from the address bus. One decoding pos-
sibility is that a write instruction to the ADC address starts a
conversion, and a read instruction reads the conversion result.
TRACK-TO-HOLD
TRANSITION
AD845
+
A1
VIN1
VIN2
V
DD
t
13
CONVST
t
1
CONVERSION
START
HOLD TO
TRACK
TRANSITION
t
12
t
10
t
2
t
4
–
CS
+V
AGND
+V
IN
V
OUT
+ 5V
+ 5REF
R1
82k
R3
5k
R2
56k
SUM
t
3
RD
t
5
t
CONV
t
11
t
7
DATA
VALID
AD586
GND
BUSY
AD707
– 3.5V
–
V
REF
+
t
6
DATA
HIGH IMPEDANCE
AD7886*
C1
10µF
C2
0.1µF
V
SS
– 5V
Figure 10. Conversion Start and Data Read Timing
Diagram
TRACK-TO-HOLD
TRANSITION
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 9. Bipolar Operation with Gain Error Adjust
CONVST
t
13
t
1
CONVERSION
START
TIMING AND CONTROL
Conversion start is controlled by the
CONVST
input (see Fig-
ures 10 and 11). A high to low going edge on the
CONVST
in-
put puts the track/hold amplifier into the hold mode. The ADC
conversion procedure does not begin until a rising
CONVST
pulse edge occurs. The width of the
CONVST
pulse low time
determines the track-to-hold settling time. The
BUSY
output,
which indicates the status of the ADC, goes low while conver-
sion is in progress. At the end of conversion
BUSY
returns high,
indicating that new data is available on the AD7886’s output
latches. The track/hold amplifier returns to the track mode at
the end of conversion and remains there until the next
CONVST
pulse. Conversion starts must not be attempted while
conversion is in progress as this will cause erroneous results.
t
5
BUSY
t
12
t
CONV
HOLD TO TRACK
TRANSITION
DATA
VALID
t
9
DATA
t
8
HIGH IMPEDANCE
Figure 11. Conversion Start and Data Read
Timing Diagram, (
CS
=
RD
= 0 V)
–8–
REV. B
AD [ ANALOG DEVICES ]
