AD589UH AD [Analog Devices], AD589UH Datasheet - Page 9

• Manufacturer Part Number: AD589UH
• Description: LC2MOS 5 us 8-Bit ADC with Track/Hold
• Manufacturer: AD [Analog Devices]

UNIPOLAR OPERATION
The basic operation for the AD7575 is in the unipolar single
supply mode. Figure 15 shows the circuit connections to achieve
this, while the nominal transfer characteristic for unipolar opera-
tion is given in Figure 16. Since the offset and full-scale errors
on the AD7575 are very small, in many cases it will not be nec-
essary to adjust out these errors. If calibration is required, the
procedure is as follows:
Offset Adjust
Offset error adjustment in single-supply systems is easily achiev-
able by means of the offset null facility of an op amp when used
as a voltage follower for the analog input signal, AIN. The op
amp chosen should be able to operate from a single supply and
allow a common-mode input voltage range that includes 0 V
(e.g., TLC271). To adjust for zero offset, the input signal
source is set to +4.8 mV (i.e., 1/2 LSB) while the op amp offset
is varied until the ADC output code flickers between 000 . . . 00
and 000 . . . 01.
Full-Scale Adjust
The full scale or gain adjustment is made by forcing the analog
input AIN to +2.445 V (i.e., Full-Scale Voltage –3/2 LSB). The
magnitude of the reference voltage is then adjusted until the
ADC output code flickers between 111 . . . 10 and 111. . . 11.
REV. B
AD589
Figure 16. Nominal Transfer Characteristic for
Unipolar Operation
+5V
+
–
A
3.3k
11111111
11111110
11111101
00000011
00000010
00000001
00000000
Figure 15. Unipolar Configuration
A
+5V
47 F
OUTPUT
CODE
+2.46V
AIN, INPUT VOLTAGE (IN TERMS OF LSBs)
MAX
1LSB
A
A
0.1 F
47 F
+1.23V
2LSBs
A
3LSBs
A
0.1 F
AIN
V
AGND
REF
AD7575
DGND
V
FULL SCALE
TRANSITION
DD
D
BUSY
FS = 2V
1LSB =
CLK
RD
CS
TP
FS –1LSB
REF
256
FS
CONTROL
INPUTS
FS
+5V
DB7–DB0
DATA OUT
+5V
D
R
100k , 1%
C
100pF, 2%
CLK
CLK
–9–
BIPOLAR OPERATION
The circuit of Figure 17 shows how the AD7575 can be config-
ured for bipolar operation. The output code provided by the
AD7575 is offset binary. The analog input voltage range is
AD7575 is in the range 0 V to +2.46 V. Figure 18 shows the
transfer function for bipolar operation. The LSB size is now
39.06 mV. Calibration of the bipolar operation is outlined be-
low. Once again, because the errors are small, it may not be
necessary to adjust them. To maintain specified performance
without the calibration, all resistors should be 0.1% tolerance
with R4 and R5 replaced by one 3.3 k resistor and R2 and R3
replaced by one 2.5 k resistor.
Offset Adjust
Offset error adjustment is achieved by applying an analog input
voltage of –4.9805 V (–FS +1/2 LSB). Resistor R3 is then
adjusted until the output code flickers between 000 . . . 00 and
000 . . . 01.
Full-Scale Adjust
Full-scale or gain adjustment is made by applying an analog
input voltage of +4.9414 V (+FS –3/2 LSB). Resistor R4 is then
adjusted until the output code flickers between 111 . . . 10 and
111. . . 11.
VOLTAGE
5 V, although the voltage appearing at the AIN pin of the
INPUT
AD589
111...111
111...110
100...010
100...001
100...000
011...111
011...110
000...001
000...000
Figure 18. Nominal Transfer Characteristic for
Bipolar Operation
+5V
10k
OUTPUT
R1
+
–
CODE
A
R8
3.3k
–FS
Figure 17. Bipolar Configuration
A
R2
2.2k
R3
500
R4
500
R5
3k
+5V
A
R6
2.5k
A
47 F
TLC271
+5V
+
–1/2LSB
A
A
2.5k
47 F
A
R7
0.1 F
+1/2LSB
A
0.1 F
V
AIN
REF
AGND
AD7575
A
FS = 5V
1LSB = FS
V
DD
DGND
+FS –1LSB
BUSY
CLK
D
256
CS
RD
AD7575
TP
+5V
+5V
AIN
D
DB7–DB0
DATA OUT
R
100k , 1%
C
100pF, 2%
CLK
CLK