CALIBRATION PROCEDURE
Connect the converter per Figure 2. Any offset/gain
calibration procedures should not be implemented until the
device is fully warmed up. To avoid interaction, adjust offset
before gain. The ranges of adjustment for the circuits in
Figure 2 are guaranteed to compensate for the ADS-935’s
initial accuracy errors and may not be able to compensate for
additional system errors.
A/D converters are calibrated by positioning their digital
outputs exactly on the transition point between two adjacent
digital output codes. This is accomplished by connecting
LED's to the digital outputs and performing adjustments until
certain LED's "flicker" equally between on and off. Other
approaches employ digital comparators or microcontrollers to
detect when the outputs change from one code to the next.
For the ADS-935, offset adjusting is normally accomplished
when the analog input is 0 minus �½ LSB (–42μV). See Table
2b for the proper bipolar output coding.
Gain adjusting is accomplished when the analog input
is at nominal full scale minus 1�½ LSB's (+2.749874V or
–5.499874V).
Note: Connect pin 5 to ANALOG GROUND (pin 4) for
operation without zero/offset adjustment. Connect pin 6 to
pin 4 for operation without gain adjustment.
Zero/Offset Adjust Procedure
1. Apply a train of pulses to the START CONVERT input (pin
12) so that the converter is continuously converting.
2. For zero/offset adjust, apply –42μV to the ANALOG INPUT
(pin 3).
3. For bipolar operation - Adjust the offset potentiometer until
the code flickers between 1000 0000 0000 0000 and 0111
1111 1111 1111 with pin 35 tied high (complementary offset
binary) or between 0111 1111 1111 1111 and 1000 0000
0000 0000 with pin 35 tied low (offset binary).
For unipolar operation - Adjust the offset potentiometer until
all outputs are 1's and the LSB flickers between 0 and 1 with
pin 35 tied high (complementary binary) or until all outputs
are 0's and the LSB flickers between 0 and 1 with pin 35 tied
low (binary).
4. For bipolar, Two's complement coding requires using
BIT 1 (MSB) (pin 29). With pin 35 tied low, adjust the trimpot
until the output code flickers between all 0’s and all 1’s.
Gain Adjust Procedure
Figure 2. Connection Diagram
1. For gain adjust, for bipolar apply +2.749874V and for unipo-
lar mode 5.499874V to the ANALOG INPUT (pin 3).
2. Adjust the gain potentiometer until all output bits are 0’s and
the LSB flickers between a 1 and 0 with pin 35 tied high
(complementary (offset) binary) or until all output bits are 1’s
and the LSB flickers between a 1 and 0 with pin 35 tied low
((offset) binary).
3. For bipolar, Two's complement coding requires using
BIT 1 (MSB) (pin 29). With pin 35 tied low, adjust the gain
trimpot until the output code flickers equally between 0111
1111 1111 1111 and 0111 1111 1111 1110.
4. To confirm proper operation of the device, vary the applied
input voltage to obtain the output coding listed in Table 2b.
Table 2a. Setting Output Coding Selection (Pin 35)
OUTPUT FORMAT
Complementary (Offset) Binary
(Offset) Binary
Complementary Two’s Complement
(Using MSB, pin 29)
Two’s Complement
(Using MSB, pin 29)
PIN 35 LOGIC LEVEL
1
0
1
0
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