ADS830

8

SBAS086A

OPA642

V

IN

R

F

402

Ω

1k

Ω

R

G

402

Ω

ADS830

R

S

24.9

Ω

1k

Ω

47pF

0.1µF

0.1

µF

IN

IN

CM

REFB

+2.0V

INT/EXT

GND

REFT

+3.0V

RSEL

+V

S

+5V

+5V

–5V

ately biased using the +2.5V common-mode voltage avail-

able at the CM pin. One-half of the amplifier (OPA2681)

buffers the REFB pin and drives the voltage divider R1, R2.

Because of the op amp’s noise gain of +2V/V, assuming

RF = RIN , the common-mode voltage (VCM) has to be re-

scaled to +1.25V, resulting in the correct DC level of +2.5V

for the signal input (IN). Any DC voltage differences be-

tween the IN and IN inputs of the ADS830 effectively

produce an offset, which can be corrected for by adjusting

the resistor values of the divider, R1 and R2. The selection

criteria for a suitable op amp should include the supply

voltage, 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.

FIGURE 3. AC-Coupling the Dual Supply Amplifier OPA642 to the ADS830 for a 2Vp-p Full Scale Input Range.

For applications requiring the driving amplifier to provide a

signal amplification, with a gain

≥ 5, consider using decom-

pensated voltage feedback op amps, such as the OPA643, or

current feedback op amps OPA681 and OPA658.

DC-Coupled with Level Shift

Several applications may require that the bandwidth of the

signal path includes DC, in which case the signal has to be

DC-coupled to the A/D converter. In order to accomplish

this, the interface circuit has to provide a DC level shift to

the analog input signal. The circuit shown in Figure 4

employs a dual op amp, A1, to drive the input of the

ADS830 and level shift the signal to be compatible with

the selected input range. With the RSEL pin tied to the

supply and the INT/EXT pin to ground, the ADS830 is

configured for a 2Vp-p input range and uses the internal

references. The complementary input (IN) may be appropri-

+V

IN

0V

–V

IN

OPA681

V

IN

R

F

402

Ω

1k

Ω

R

G

402

Ω

ADS830

R

S

39

Ω

47pF

0.1µF

IN

IN

CM

INT/EXT

GND

REFT

+3.0V

1k

Ω

V

CM = +2.5VDC

REFB

+2.0V

0.1

µF

0.1µF

RSEL

+V

S

+5V

+5V

FIGURE 2. AC-Coupled Input Configuration for a 2Vp-p Full-Scale Range and a Common-Mode Voltage, VCM, at +2.5V

Derived from the Internal Top (REFT) and Bottom Reference (REFB). The OPA680 can be used in place of the

OPA681 if a voltage feedback amplifier is preferred.