DC-Coupled, Differential Driver with Level Shift
Several applications will require that the bandwidth of the
signal path include DC, in which case, the signal has to be
DC-coupled to the ADC. An op amp based interface circuit
can be configured to scale and level shift the input signal to
be compatible with the selected input range of the ADC. The
circuit shown in Figure 5 employs a dual op amp, OPA2681,
to drive the input of the ADS2806 differentially. The single-
supply, general-purpose op amp OPA234 is added to buffer
the common-mode voltage of +2.5V, available at the CM pin,
and apply it to the input of the driver amplifier. This sets the
correct DC voltage to bias the inputs of the ADS2806. It
should be noted that any DC voltage differences between the
IN and IN inputs of the ADS2806 will result in an offset error.
can be either hardwired to ground or left unconnected, which
will default the converter to a 2Vp-p full-scale input range
(FSR). While set for the 2Vp-p range, the top and bottom
reference voltages will be REFT = +3.0V and REFB = +2.0V.
Switching to the 3Vp-p range changes those voltages to
REFT = +3.25V and REFB = +1.75V. The reference buffers
can be utilized to supply up to 1mA/channel (2mA total, sink
and source) to external circuitry. To ensure proper operation
with any reference configuration, it is necessary to provide
solid bypassing at all reference pins in order to keep the clock
feedthrough to a minimum, as shown in Figure 6. Good
performance requires using 0.1µF low inductance capacitors.
All bypassing capacitors should be located as close to their
respective pins as possible.
Using the OPA2681, this circuit can be operated either with
a single or a dual ±5V supply.
REFERENCE OPERATION
1/2
The internal reference consists of a bandgap voltage refer-
ence, the drivers for the top and bottom reference, and the
resistive reference ladder. References are internally con-
nected, e.g.: REFTA is connected to REFTB, and REFBA is
connected to REFBB. The bandgap reference circuit includes
logic functions that allow setting the analog input swing of the
ADS2806 to a differential full-scale range of either 2Vp-p or
3Vp-p by simply tying the SEL pin to a LOW or HIGH
potential, respectively. While operating the ADS2806 in the
external reference mode, the buffer amplifiers for REFT and
REFB are disabled. The ADS2806 has an internal 50kΩ pull-
down resistor at the range select pin (SEL). Therefore, this pin
ADS2806
REFT
CM
REFB
+
+
+
10µF
0.1µF
10µF
0.1µF
10µF
0.1µF
FIGURE 6. Recommended Bypassing for the Reference Pins.
499Ω
249Ω
24.9Ω
VIN
1/2
IN
OPA2681
22pF
499Ω
499Ω
249Ω
1/2
ADS2806Y
IN
249Ω
24.9Ω
CM
1/2
OPA2681
22pF
499Ω
24.9Ω
OPA234
249Ω
0.1µF
0.1µF
0.1µF
1kΩ
One Channel of Two
FIGURE 5. DC-Coupled Input Driver with Level Shifting.
ADS2806
SBAS178B
13
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