Datasheet
ADS803
9
SBAS074B
www.ti.com
V
IN
IN
IN
CM
22Ω
22Ω
100pF
R
T
100pF
+
4.7µF
0.1µF
ADS803
1:n
0.1µF
R
G
REFERENCE OPERATION
Integrated into the ADS803 is a bandgap reference circuit
including logic that provides either a +1V or +2.5V reference
output by simply selecting the corresponding pin-strap con-
figuration. Different reference voltages can be generated by
the use of two external resistors, which will set a different
gain for the internal reference buffer. For more design flexibil-
ity, the internal reference can be shut off and an external
reference voltage used. Table I provides an overview of the
possible reference options and pin configurations.
FIGURE 3. Transformer-Coupled Input
INPUT
FULL-SCALE REQUIRED
MODE RANGE V
REF
CONNECT TO
Internal 2Vp-p +1V SEL V
REF
Internal 5Vp-p +2.5V SEL GND
Internal 2V ≤ FSR < 5V 1V < V
REF
< 2.5V R
1
V
REF
and SEL
FSR = 2 x V
REF
V
REF
= 1 + (R
1
/R
2
)R
2
SEL and GND
External 1V < FSR < 5V 0.5V < V
REF
< 2.5V SEL +V
S
V
REF
Ext. V
REF
TABLE I. Selected Reference Configuration Examples.
A simple model of the internal reference circuit is shown in
Figure 4. The internal blocks are a 1V-bandgap voltage
reference, buffer, the resistive reference ladder, and the
drivers for the top and bottom reference that supply the
necessary current to the internal nodes. As shown, the
output of the buffer appears at the V
REF
pin. The full-scale
input span of the ADS803 is determined by the voltage at
V
REF
, according to Equation 1:
Full-Scale Input Span = 2 • V
REF
(1)
Note that the current drive capability of this amplifier is limited to
approximately 1mA and should not be used to drive low loads.
The programmable reference circuit is controlled by the voltage
applied to the select pin (SEL). Refer to Table I for an overview.
FIGURE 4. Equivalent Reference Circuit.
800Ω
800Ω
REFB
CM
Reference
Driver
Bandgap
and Logic
REFT
V
REF
SEL
1V
DC
Resistor Network
and Switches
Disable
Switch
to A/D
to A/D
ADS803