ADS822, ADS825

9

SBAS069B

From the Internal Top (REFT) and Bottom References (REFB).

FIGURE 3. AC-Coupling the Dual Supply Amplifier, OPA642, to the ADS822 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, like the OPA686, or

current-feedback op amps like the OPA691.

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 to the analog

input signal. See Figure 4 for a circuit that employs a dual op

amp, A1, to drive the input of the ADS822 and ADS825, and

level shifts 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 ADS822 and ADS825 are configured for a

2Vp-p input range and use the internal references. The

complementary input (IN) may be appropriately biased using

the +2.5V common-mode voltage available at the CM pin.

One half of amplifier A1 buffers the REFB pin and drives the

has to be re-scaled to +1.25V. This results in the correct DC

level of +2.5V for the signal input (IN). Any DC voltage

differences between the IN and IN inputs of the ADS822 and

ADS825 effectively produces an offset, which can be cor-

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 inter-

change the IN and IN connections.

+V

IN

0V

–V

IN

OPA690

V

IN

R

F

402

Ω

1.62k

Ω

R

G

402

Ω

ADS822

ADS825

R

S

50

Ω

10pF

0.1

µF

IN

IN

CM

INT/EXT

GND

REFT

+3.5V

1.62k

Ω

50

Ω

V

REFB

+1.5V

0.1

µF

0.1

µF

RSEL

+V

S

+5V

+5V

OPA642

V

IN

R

F

402

Ω

1.62k

Ω

R

G

402

Ω

ADS822

ADS825

R

S

24.9

Ω

1.62k

Ω

100pF

0.1

µF

0.1

µF

IN

IN

CM

REFB

+1.5V

INT/EXT

GND

REFT

+3.5V

RSEL

+V

S

+5V

+5V

–5V