ered when choosing the accuracy and drift specifications of
the external resistors. In most applications, 1% metal-film
resistors will be sufficient.
To further analyze the effects of removing any combination of
the external resistors, consider Figure 9. The combination of
the external and the internal resistors form a voltage divider
which reduces the input signal to a 0.3125V to 2.8125V input
range at the Capacitor Digital-to-Analog Converter (CDAC).
The internal resistors are laser trimmed to high relative accu-
racy to meet full scale specifications. The actual input imped-
ance of the internal resistor network looking into pin 1 or pin
3 however, is only accurate to ±20% due to process variations.
This should be taken into account when determining the
effects of removing the external resistors.
The external resistors, see Figure 7b, 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. Not using the external
resistors will result in offset and gain errors in addition to
those listed in the electrical characteristics section. Offset
refers to the equivalent voltage of the digital output when
converting with the input grounded. A positive gain error
occurs when the equivalent output voltage of the digital
output is larger than the analog input. Refer to Table VII for
nominal ranges of gain and offset errors with and without the
external resistors. Refer to Figure 8 for typical shifts in the
transfer functions which occur when the external resistors
are removed.
REFERENCE
The ADS7807 can operate with its internal 2.5V reference or
an external reference. By applying an external reference to
pin 5, the internal reference can be bypassed; REFD (pin 26)
OFFSET ERROR
GAIN ERROR
WITH RESISTORS
RANGE (mV)
WITHOUT RESISTORS
WITH RESISTORS
RANGE (% FS)
WITHOUT RESISTORS
INPUT
RANGE (V)
RANGE (mV)
TYP (mV)
RANGE (% FS)
TYP
±10
–10 ≤ BPZ ≤ 10
0 ≤ BPZ ≤ 35
15
–7.5
–6
–0.4 ≤ G ≤ 0.4
–0.3 ≤ G ≤ 0.5
+0.05
+0.05
0.15 ≤ G(1) ≤ 0.15
–0.1 ≤ G(1) ≤ 0.2
0 to 5
0 to 4
–3 ≤ UPO ≤ 3
–3 ≤ UPO ≤ 3
–12 ≤ UPO ≤ –3
–0.4 ≤ G ≤ 0.4
0.15 ≤ G(1) ≤ 0.15
–1.0 ≤ G ≤ 0.1
–0.55 ≤ G(1) ≤ –0.05
–0.2
–0.2
–10.5 ≤ UPO ≤ –1.5
–0.4 ≤ G ≤ 0.4
–0.15 ≤ G(1) ≤ 0.15
–1.0 ≤ G ≤ 0.1
–0.55 ≤ G(1) ≤ –0.05
–0.2
–0.2
NOTE: (1) High Grade.
TABLE VII. Range of Offset and Gain Errors With and Without External Resistors.
(a) Bipolar
(b) Unipolar
Digital Output
Digital Output
+Full-Scale
+Full-Scale
Analog Input
–Full-Scale
Analog Input
–Full-Scale
Typical Transfer Functions
With External Resistors
Typical Transfer Functions
Without External Resistors
FIGURE 8. Typical Transfer Functions With and Without External Resistors.
ADS7807
14
SBAS022C
www.ti.com
BB [ BURR-BROWN CORPORATION ]
