MT-074

DRIVING PRECISION 16- AND 18-BIT DIFFERENTIAL INPUT ADCs

Figure 3 shows the ADA4941-1 driving the 18-bit PulSAR family of ADCs which have switched

capacitor inputs. This is a common application where the signal is single-ended and bipolar and

the ADC input is differential. Because of the high resolution, the drive amplifier must have low

distortion, low noise, and high dc accuracy, as well as the capability of performing the single-

ended to differential conversion. The ADA4941-1 is a low power (2.2 mA @ 3.3 V), low noise

(10.2 nV/

√Hz @ 1 kHz), low distortion (110 dBc @ 100 kHz) differential driver for ADCs up to

18 bits. Small signal bandwidth is 31 MHz. The amplifier has rail-to-rail output, high input

impedance, and a user-adjustable gain.

The ADA4941-1 consists of two op amps. The lower one in the figure is configured as a non-

commited non-inverting buffer (with external feedback resistor) and drives an inverting

amplifier. The feedforward and feedback resistors for the inverting amplifier are included in the

IC. Although there is extra phase shift and delay through the inverting amplifier, this does not

introduce significant error at the frequencies of interest (up to 1 MHz or 2 MHz).

+5V

+2.1V

+1.75V

9.53k

Ω

10.0k

Ω

8.45k

Ω

0.1µF

0.1µF

11.3k

Ω

4.02k

Ω

806

Ω

ADR444

+5V

0.1µF

REF

+5V

VDD

IN+

IN–

+

+

–

–

V

± 10V

+2.1V +/– 2V

+2.1V – /+ 2V

ADA4941-1

41.2

Ω

41.2

Ω

3.9nF

3.9nF

AD7690, 400kSPS

AD7691, 250kSPS

18-BIT

PulSAR

ADCs

LPF CUTOFF = 1MHz

V

R

R

0.1µF

V

INPUT RANGE=

8.192V p-p DIFF.

10.2nV/

√Hz

After filter, noise = 13µV rms due to amp

Signal = 8V p-p differential

SNR = 107dB @ ADC input

SNR = 100dB

For AD7690

Figure 3: ADA4941-1 Driving AD7690 18-Bit

PulSAR® ADC in +5V Application

In this application, the two resistor dividers set the output common-mode voltage of the

ADA4941-1 to +2.1 V so that the output only has to go to within 100 mV of ground. This allows

sufficient headroom for the rail-to-rail output stages of the amplifier and allows the entire circuit

to operate on a single +5 V supply.

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