each amplifier. This pulls a DC bias current out of the output
stage of the amplifier. It is set to approximately 5mA, see
Figure 3, but will vary depending on the amplifier used.
top and bottom references (REFT, REFB) provide an output
voltage of +3.0V and +2.0V, respectively. The CM output of
the ADS2806 is used to bias the inputs of the driving
amplifiers. Using the OPA2681 on a single +5V supply, its
ideal common-mode point is +2.5V, which coincides with
the recommended common-mode input level for the
ADS2806, thus eliminating the need for coupling capacitors
between the amplifiers and the converter.
Single-Ended, AC-Coupled, Dual-Supply Interface
The circuit provided in Figure 4 shows typical connections
for using the ADS2806 in a single-ended input configura-
tion. The bias requirements for AC-coupling are provided by
a single resistor to the CM output lead. The single-ended
mode of operation should be considered for ease of inter-
face complexity and applications where the dynamic perfor-
mance can be compromised. The series resistor RS, along
with the shunt capacitance, provide the means to adjust the
bandwidth and optimize the performance towards good
signal-to-noise ratio. In addition, the amplifier configuration
can be easily modified for an anti-aliasing filter based on a
2nd-order Sallen-Key or Multiple-Feedback topology.
The addition of a small series resistor (RS) between the
output of the op amps and the input of the ADS2806 will be
beneficial in almost all interface configurations. It will de-
couple the op amp’s output from the capacitive load and
avoid gain peaking that can result in increased noise. For
best spurious and distortion performance, the resistor value
should be kept below 100Ω. Furthermore, the series resis-
tor, in combination with the shunt capacitor, establishes a
passive low-pass filter limiting the bandwidth for the wideband
noise, thus improving the SNR. The spurious-free dynamic
range of this single-supply front end is limited by the 2nd-
harmonic distortion. An improvement of several dB may be
realized by adding a pull-down resistor (RP) at the output of
The interface example, shown in Figure 4, operates with the
full-scale range of the ADS2806 set to 2Vp-p, leaving
sufficient headroom for the output of the OPA642 to drive
the converter and maintain low signal distortion.
+5V
RS
16.5Ω
0.1µF
VIN
IN
OPA642
68pF
1/2
ADS2806Y
–5V
RF
402Ω
CM
1.82kΩ
IN
0.1µF
RG
402Ω
One Channel of Two
FIGURE 4. AC-Coupling the Dual-Supply Amplifier OPA642 to the ADS2806 for a 2Vp-p Full-Scale Input Range.
ADS2806
SBAS178B
12
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