ADS825E/1K 参数 Datasheet PDF下载

1.62kΩ

+5V

V

CM

+2.5V

+5V

0.1µF

V

IN

OPA680

10pF

+V

IN

0V

–V

IN

R

F

402Ω

CM

R

G

402Ω

0.1µF

IN

0.1µF

INT/EXT

GND

ADS822

ADS825

50Ω

R

S

50Ω

IN

REFB

+1.5V

REFT

+3.5V

RSEL

+V

S

1.62kΩ

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

CM

, at +2.5V Derived

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

+5V

1.62kΩ

+5V

V

IN

OPA642

100pF

–5V

R

F

402Ω

1.62kΩ

CM

IN

R

G

402Ω

0.1µF

REFB

+1.5V

ADS822

ADS825

R

S

24.9Ω

0.1µF

IN

REFT

+3.5V

RSEL

+V

S

INT/EXT

GND

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 OPA643, or

current-feedback op amps like the OPA681 and OPA658.

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 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

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 voltage

divider R

1

, R

2

. Due to the op amp’s noise gain of

+2V/V, assuming R

F

= R

IN

, the common-mode voltage

(V

CM

) 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 corrected for by adjusting the resistor values of the

divider, R

1

and R

2

. 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 in-

verted. To re-establish the original signal polarity, it is

always possible to interchange the IN and IN connections.

®

9

ADS822, ADS825