Datasheet

ADS7805
10
SBAS020D
www.ti.com
FIGURE 6. Circuit Diagram With and Without External Resistors.
a) ±10V With Hardware
Trim
b) ±10V Without Hardware
Trim
NOTE: Use 1% metal film resistors.
WITH WITHOUT
EXTERNAL EXTERNAL
RESISTORS RESISTORS UNITS
BP0 –10 < BPO < 10 –50 < BPO < –15 mV
–30 < BPO < 30 –150 < BPO < –45 LSBs
Gain –0.5 < error < 0.5 –2 < error < –1 % of FSR
Error
TABLE V. Offset and Gain Errors With and Without External
Resistors.
200Ω
1
2
3
4
5
AGND2
CAP
REF
AGND1
V
IN
+
2.2µF
+5V
50kΩ
50kΩ
33.2kΩ
576kΩ
+
2.2µF
Gain
±10V
Offset
200Ω
1
2
3
4
5
AGND2
CAP
REF
AGND1
V
IN
+
2.2µF
33.2kΩ
±10V
+
2.2µF
INPUT RANGES
The ADS7805 offers a standard ±10V input range. Figure 6
shows the necessary circuit connections for the ADS7805
with and without hardware trim. Offset and full-scale error
(1)
specifications are tested and specified with the fixed resistors
shown in Figure 6b. Adjustments for offset and gain are
described in the “Calibration” section of this data sheet.
The offset and gain are adjusted internally to allow external
trimming with a single supply. The external resistors com-
pensate for this adjustment and can be left out if the offset
and gain will be corrected in software (refer to the “Calibra-
tion” section).
The nominal input impedance of 23kΩ results from the combi-
nation of the internal resistor network shown on the front page
of the product data sheet and the external resistors. The input
resistor divider network provides inherent overvoltage protec-
tion ensured 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 influence relative to the internal
resistors, and tighter tolerances are not required.
NOTE: (1) Full-scale error includes offset and gain errors measured at both +FS
and –FS.
CALIBRATION
The ADS7805 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.
HARDWARE CALIBRATION
To calibrate the offset and gain of the ADS7805, install the
proper resistors and potentiometers as shown in Figure 6a.
The calibration range is ±15mV for the offset and ±60mV for
the gain.
SOFTWARE CALIBRATION
To calibrate the offset and gain of the ADS7805 in software, no
external resistors are required. See the “No Calibration” sec-
tion for details on the effects of the external resistors. Range of
offset and gain errors with and without external resistors is
shown in Table V.
NO CALIBRATION
Figure 6b shows circuit connections. The external resistors
shown in Figure 6b may not be necessary in some applica-
tions. 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 ADS7805
will be bound by the shaded region (see 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.