ADS807

12

SBAS072A

www.ti.com

OPA642

V

IN

R

F

402

Ω

R

G

402

Ω

ADS807E

R

S

16.5

Ω

68pF

0.1µF

0.1

µF

IN

IN

CM

+5V

–5V

1.82k

Ω

Single-Ended, AC-Coupled, Dual-Supply Interface

The circuit provided in Figure 4 shows typical connections for

using the ADS807 in a single-ended input configuration. 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 interface com-

plexity and applications where the dynamic performance 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 interface example shown in Figure 4 operates with the

full-scale range of the ADS807 set to 2Vp-p, leaving suffi-

cient headroom for the output of the OPA642 to drive the

converter and maintain low signal distortion.

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 A/D converter. 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

A/D converter. The circuit shown in Figure 5 employs a dual

op amp, OPA2681, to drive the input of the ADS807 differ-

entially. 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 ADS807. It should be noted that any DC voltage

differences between the IN and IN inputs of the ADS807 will

result in an offset error.

Using the OPA2681, this circuit can be operated either with

a single or a dual

±5V supply.

FIGURE 4. AC-Coupling the Dual-Supply Amplifier OPA642 to the ADS807 for a 2Vp-p Full-Scale Input Range.

FIGURE 5. DC-Coupled Input Driver with Level Shifting.

V

IN

ADS807E

IN

IN

CM

499

Ω

249

Ω

24.9

Ω

24.9

Ω

249

Ω

OPA2681

0.1

µF

0.1

µF

0.1

µF

499

Ω

499

Ω

499

Ω

249

Ω

249

Ω

24.9

Ω

1k

Ω

22pF

22pF

OPA234