Datasheet

ADS850
SBAS154C
10
www.ti.com
V
IN
2Vp-p
0
+1V
–1V
R
F
R
IN
NOTE: R
F
= R
IN
, G = –1
+V
S
ADS850
R
S
24.9Ω
2kΩ
100pF
10µF
+
+2.5V
R
2
R
1
2kΩ
IN
IN
REFB
(+1V)
V
REF
SEL
REFT
0.1µF
0.1µF
OPA691
+V
S
FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-level Shift.
OPA642
V
IN
+V
IN
0V
–V
IN
R
F
402Ω
R
G
402Ω
ADS850
R
S
24.9Ω
2kΩ2kΩ
2kΩ
2kΩ
100pF
0.1µF
0.1µF2Vp-p
IN
IN
(+2V)
REFB
(+1V)
VREF
SEL
REFT
(+3V)
+5V
–5V
+2.5V
DC
FIGURE 1. AC-Coupled Input Configuration for 2Vp-p Input Swing and Common-Mode Voltage at +2.5V Derived from Internal
Top and Bottom Reference.
DC-COUPLED WITHOUT LEVEL SHIFT
In some applications the analog input signal may already be
biased at a level which complies with the selected input
range and reference level of the ADS850. In this case, it is
only necessary to provide an adequately low source imped-
ance to the selected input, IN or
IN . Always consider wideband
op amps since their output impedance will stay low over a
wide range of frequencies. For those applications requiring
the driving amplifier to provide a signal amplification, with a
gain ≥ 3, consider using the decompensated voltage feed-
back op amp OPA686.
DC-COUPLED WITH LEVEL SHIFT
Several applications may require that the bandwidth of the
signal path include DC, in which case the signal has to be DC-
coupled to the ADC. In order to accomplish this, the interface
circuit has to provide a DC-level shift. The circuit shown in
Figure 2 employs an op amp, OPA681, to sum the ground
centered input signal with a required DC offset. The ADS850
typically operates with a +2.5V common-mode voltage, which
is established at the center tap of the ladder and connected
to the
IN input of the converter. The OPA681 operates in
inverting configuration. Here resistors R
1
and R
2
set the DC-
bias level for the OPA691. Because of the op amp’s noise
gain of +2V/V, assuming R
F
= R
IN
, the DC offset voltage
applied to its noninverting input has to be divided down to
+1.25V, resulting in a DC output voltage of +2.5V. DC voltage
differences between the IN and
IN inputs of the ADS850
effectively will produce an offset, which can be corrected for
by adjusting the values of resistors R
1
and R
2
. The bias
current of the op amp may also result in an undesired offset.
The selection criteria of the appropriate op amp should
include the input bias current, output voltage swing, distortion
and noise specification. Note that in this example the overall
signal phase is inverted. To re-establish the original signal
polarity, it is always possible to interchange the IN and
IN
connections.