INPUT RANGES
HARDWARE CALIBRATION
The ADS7804 offers a standard ±10V input range. Figure 6
shows the necessary circuit connections for the ADS7804
with and without hardware trim. Offset and full scale error(1)
specifications are tested and guaranteed with the fixed
resistors shown in Figure 6b. Adjustments for offset and
gain are described in the Calibration section of this data
sheet.
To calibrate the offset and gain of the ADS7804, install the proper
resistors and potentiometers as shown in Figure 6a. The calibra-
tion range is ±15mV for the offset and ±60mV for the gain.
SOFTWARE CALIBRATION
To calibrate the offset and gain of the ADS7804 in software,
no external resistors are required. See the No Calibration
section for details on the effects of the external resistors.
Refer to Table V for range of offset and gain errors with and
without external resistors.
The offset and gain are adjusted internally to allow external
trimming with a single supply. The external resistors compen-
sate for this adjustment and can be left out if the offset and
gain will be corrected in software (refer to the Calibration
section).
NO CALIBRATION
The nominal input impedance of 23kW results from the
combination of the internal resistor network shown on the
front page of the product data sheet and the external resis-
tors. The input resistor divider network provides inherent
overvoltage protection guaranteed to at least ±25V. The 1%
resistors used for the external circuitry do not compromise
the accuracy or drift of the converter. They have little influ-
ence relative to the internal resistors, and tighter tolerances
are not required.
See Figure 6b for circuit connections. The external resistors
shown in Figure 6b may not be necessary in some applications.
These resistors provide compensation for an internal adjustment
of the offset and gain which allows calibration with a single supply.
The nominal transfer function of the ADS7804 will be bound by
the shaded region seen in Figure 7 with a typical offset of –30mV
and a typical gain error of –1.5%. Refer to Table V for range of
offset and gain errors with and without external resistors.
NOTE: (1) Full scale error includes offset and gain errors measured at both
+FS and –FS.
WITH
EXTERNAL
RESISTORS
WITHOUT
EXTERNAL
RESISTORS
UNITS
CALIBRATION
BPZ
–10 < BPZ < 10
–2 < BPZ < 2
–45 < BPZ < 5
–8 < BPZ < 1
mV
LSBs
The ADS7804 can be trimmed in hardware or software. The
offset should be trimmed before the gain since the offset
directly affects the gain. To achieve optimum performance,
several iterations may be required.
Gain
Error
–0.5 < error < 0.5
–0.6 < error < –0.55
–0.45 < error < –0.3(1)
% of FSR
–0.25 < error < 0.25(1)
NOTE: (1) High Grade.
TABLE VII. Bipolar Offset and Gain Errors With and Without
External Resistors.
a)
±10V With Hardware
b)
±10V Without Hardware
Trim
Trim
200Ω
200Ω
1
2
3
4
5
1
±10V
±10V
VIN
VIN
2
33.2kΩ
AGND1
REF
AGND1
+5V
2.2µF
576kΩ
2.2µF
2.2µF
2.2µF
+
+
33.2kΩ
+
+
3
4
5
REF
50kΩ
Gain
Offset
CAP
CAP
50kΩ
AGND2
AGND2
NOTE: Use 1% metal film resistors.
FIGURE 6. Circuit Diagram With and Without External Resistors.
ADS7804
8
SBAS019A
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