Datasheet

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APPLICATION INFORMATION
CALIBRATION
Hardware Calibration
Software Calibration
No Calibration
1
2
3
4
5
AGND2
CAP
REF
AGND1
V
IN
+
+5 V
+
Gain
Offset
1
2
3
4
5
AGND2
CAP
REF
AGND1
V
IN
+
+
±10 V
200
33.2 k
50 k
50 k
2.2 µF
2.2 µF
2.2 µF
2.2 µF
200
33.2 k
±10 V
(a) ±10 V With Hardware Trim (b) ±10 V Without Hardware Trim
Note: Use 1% metal film resistors.
576 k
REFERENCE
REF
ADS8505
SLAS180B SEPTEMBER 2005 REVISED JUNE 2007
The ADS8505 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.
To calibrate the offset and gain of the ADS8505, install the proper resistors and potentiometers as shown in
Figure 28 (a).
To calibrate the offset and gain of the ADS8505 in software, no external resistors are required. See the No
Calibration section for details on the effects of the external resistors.
See Figure 28 (b) for circuit connections. The external resistors shown in Figure 28 (b) 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.
Figure 28. Circuit Diagram With and Without External Resistors
The ADS8505 can operate with its internal 2.5-V reference or an external reference. By applying an external
reference to pin 5, the internal reference can be bypassed. The reference voltage at REF is buffered internally
with the output on CAP (pin 4).
The internal reference has an 8 ppm/ ° C drift (typical) and accounts for approximately 20% of the full-scale error
(FSE = ± 0.5%).
REF (pin 3) is an input for an external reference or the output for the internal 2.5-V reference. A 2.2- µ F capacitor
should be connected as close to the REF pin as possible. The capacitor and the output resistance of REF create
a low-pass filter to bandlimit noise on the reference. Using a smaller value capacitor introduces more noise to
the reference degrading the SNR and SINAD. The REF pin should not be used to drive external ac or dc loads.
The range for the external reference is 2.3 V to 2.7 V and determines the actual LSB size. Increasing the
reference voltage increases the full-scale range and the LSB size of the converter which can improve the SNR.
15
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