Datasheet

ADS7806
14
SBAS021B
www.ti.com
ered when choosing the accuracy and drift specifications of
the external resistors. In most applications, 1% metal-film
resistors will be sufficient.
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 resis-
tors 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.
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 specifications. The actual input impedance 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.
REFERENCE
The ADS7806 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)
WITH RESISTORS WITHOUT RESISTORS WITH RESISTORS WITHOUT RESISTORS
RANGE (V) RANGE (mV) RANGE (mV) TYP (mV) RANGE (% FS) RANGE (% FS) TYP
±10 –10 ≤ BPZ ≤ 10 0 ≤ BPZ ≤ 35 +15 –0.4 ≤ G ≤ 0.4 –0.3 ≤ G ≤ 0.5 +0.05
0.15 ≤ G
(1)
≤ 0.15 –0.1 ≤ G
(1)
≤ 0.2 +0.05
0 to 5 –3 ≤ UPO ≤ 3 –12 ≤ UPO ≤ –3 –7.5 –0.4 ≤ G ≤ 0.4 –1.0 ≤ G ≤ 0.1 –0.2
0.15 ≤ G
(1)
≤ 0.15 –0.55 ≤ G
(1)
≤ –0.05 –0.2
0 to 4 –3 ≤ UPO ≤ 3 –10.5 ≤ UPO ≤ –1.5 –6 –0.4 ≤ G ≤ 0.4 –1.0 ≤ G ≤ 0.1 –0.2
–0.15 ≤ G
(1)
≤ 0.15 –0.55 ≤ G
(1)
≤ –0.05 –0.2
NOTE: (1) High Grade.
OFFSET ERROR GAIN ERROR
TABLE VII. Range of Offset and Gain Errors With and Without External Resistors
FIGURE 8. Typical Transfer Functions With and Without External Resistors.
INPUT
Digital Output
(b) Unipolar
–Full-Scale
+Full-Scale
Analog Input
Digital Output
(a) Bipolar
–Full-Scale
+Full-Scale
Analog Input
Typical Transfer Functions
With External Resistors
Typical Transfer Functions
Without External Resistors