Datasheet

ADS850
SBAS154C
11
www.ti.com
V
IN
IN
IN
REFB
22Ω
22Ω
100pF
0.1µF
0.1µF
R
T
100pF
0.1µF
REFT
ADS850
2kΩ
2kΩ
1:n
0.1µF
R
G
FIGURE 3. Transformer-Coupled Input.
SINGLE-ENDED-TO-DIFFERENTIAL
CONFIGURATION (TRANSFORMER COUPLED)
In order to select the best suited interface circuit for the
ADS850, the performance requirements must be known. If
an ac-coupled input is needed for a particular application, the
next step is to determine the method of applying the signal;
either single-ended or differentially. The differential input
configuration may provide a noticeable advantage of achiev-
ing good SFDR performance based on the fact that in the
differential mode, the signal swing can be reduced to half of
the swing required for single-ended drive. Secondly, by
driving the ADS850 differentially, the even-order harmonics
will be reduced. Figure 3 shows the schematic for the
suggested transformer-coupled interface circuit. The resistor
across the secondary side (R
T
) should be set to get an input
impedance match (e.g., R
T
= n
2
• R
G
).
REFERENCE OPERATION
Integrated into the ADS850 is a bandgap reference circuit
including logic that provides either a +1V or +2V reference
output, by simply selecting the corresponding pin-strap con-
figuration. For more design flexibility, 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.
INPUT
MODE RANGE SEL V
REF
REFB REFT
Internal 2Vp-p V
REF
SEL NC NC
Internal 4Vp-p GND NC NC NC
External 2V < FSR < 4V +V
S
1V < FSR < 2V NC NC
External (REFB – REFT) • 2+V
S
GND 1.5V < REFB < 2V 2V < REFT <3.5V
TABLE I. Selected Reference Configuration Examples.
800Ω
800Ω
REFB
CM
Reference
Driver
Bandgap
and Logic
REFT
V
REF
SEL
1V
DC
Resistor Network
and Switches
Disable
Switch
to ADC
to ADC
ADS850
FIGURE 4. Equivalent Reference Circuit.
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 ADS850 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 about 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.