Datasheet

ADS805
10
SBAS073B
www.ti.com
FIGURE 5. Recommended Reference Bypassing Scheme.
FIGURE 6. Alternative Circuit to Generate Common-Mode Voltage.
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 which 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 ADS805 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.
The top reference (REFT) and the bottom reference (REFB)
are brought out mainly for external bypassing. For proper
operation with all reference configurations, it is necessary to
provide solid bypassing to the reference pins in order to keep
the clock feedthrough to a minimum. Figure 5 shows the
recommended decoupling network.
In addition, the Common-Mode Voltage (CMV) may be used
as a reference level to provide the appropriate offset for the
driving circuitry. However, care must be taken not to appre-
ciably load this node, which is not buffered and has a high
impedance. An alternate method of generating a common-
mode voltage is given in Figure 6. Here, two external preci-
sion resistors (tolerance 1% or better) are located between
the top and bottom reference pins. The common-mode level
will appear at the midpoint. The output buffers of the top and
bottom reference are designed to supply approximately 2mA
of output current.
IN
IN
REFT
R
1
CM
R
2
0.1µF
0.1µF
ADS805
REFB
ADS805
CMREFB
0.1µF
0.1µF
V
REF
0.1µF
10µF
+
0.1µF
REFT
0.1µF
10µF
+
FIGURE 4. Equivalent Reference Circuit.
800Ω
800Ω
REFB
CM
Reference
Driver
Bandgap
and Logic
REFT
V
REF
SEL
1V
DC
ADS805
Resistor Network
and Switches
Disable
Switch
to A/D
Converter
to A/D
Converter