ADS901

9

SBAS054A

R

1

1k

Ω

OPA680

V

IN

R

F

402

Ω

V

CM

C

1

0.1µF

0.1µF

IN

CM

+5V

R

S

50

Ω

+3V

R

G

402

Ω

C

G

0.1µF

402

Ω

22pF

ADS901

FIGURE 4. DC-Coupled Interface Circuit for +3V Single-Supply Operation.

dual supplies (OPA650, OPA658). The mid-point reference

filter with the –3dB frequency set at

(2)

and can be set freely. The values shown correspond to a

frequency of 1.6kHz.

Figure 3 depicts a circuit that can be used in single-supply

applications. The mid-reference voltage biases the op amp

up to the appropriate common-mode voltage, for example

the non-inverting op amp input is set to +1V/V. As a result

the transfer function is modified to

(3)

a high-pass filter. At the same time the input impedance is

the capacitive load to avoid gain peaking or even oscillation.

It can also be used to establish a defined bandwidth to reduce

the wideband noise. The recommended value is usually

between 10

Ω and 100Ω.

DC-COUPLED INTERFACE CIRCUIT

Many systems are now requiring +3V single supply capabil-

ity of both the A/D converter and its driver. Figure 4 shows

an example for DC-coupled configuration operating solely

on a +3V supply voltage. The OPA632 provides excellent

performance in this demanding application. Its wide input

and output voltage ranges, an low distortion, supports the

ADS901 well. The OPA632 is configured for a gain of +2.

The 374

Ω and 2.26kΩ resistors at the input level-shift V

IN

set to match to a 50

Ω source impedance. The input level-

while delivering an output voltage of 1V to 2V into the

ADS901. Both the OPA632 and ADS901 have a power-

down function pin with the same polarity for those systems

the need to conserve power.

EXTERNAL REFERENCE

The ADS901 requires external references on pin 22 (REFT)

and 24 (REFB). Internally those pins are connected through a

resistor ladder, which has a nominal resistance of 4k

Ω

(

±15%). In order to establish a correct voltage drop across the

ladder the external reference circuit must be able to typically

supply 250

µA of current. With this current the full-scale input

range of the ADS901 is set between +1V and +2V, or 1Vp-p.

In general, the voltage drop across REFT and REFB deter-

mines the input full-scale range (FSR) of the ADS901. Equa-

tion (4) can be used to calculate the span.

FSR = REFT - REFB

(4)

Depending on the application, several options are possible to

supply the external reference voltages to the ADS901 without

degrading the typical performance.

FIGURE 3. Interface Circuit. Example using the voltage feedback amplifier OPA680.

OPA632

V

IN

750

Ω

562

Ω

2.26k

Ω

374

Ω

22pF

+3V

100

Ω

Pwrdn

DIS

Disable

+3V

ADS901

10-Bit

20Msps

57.6

Ω