ADA4938-1/ADA4938-2

Data Sheet

Rev. B | Page 24 of 26

HIGH PERFORMANCE ADC DRIVING

The ADA4938-1/ADA4938-2 are ideally suited for dc-coupled

baseband applications. The circuit in Figure 69 shows a front-end

connection for an ADA4938-1/ADA4938-2 driving an AD9446,

16-bit, 80 MSPS ADC. The AD9446 achieves its optimum

performance when it is driven differentially. The ADA4938-1/

ADA4938-2 eliminate the need for a transformer to drive the

ADC, performs a single-ended-to-differential conversion,

buffers the driving signal, and provides appropriate level

shifting for dc coupling.

The ADA4938-1/ADA4938-2 are configured with a single 10 V

supply and unity gain for a single-ended input to differential

output. The 61.9 Ω termination resistor, in parallel with the

single-ended input impedance of 267 Ω, provides a 50 Ω

termination for the source. The additional 26 Ω (226 Ω total) at the

inverting input balances the parallel impedance of the 50 Ω

source and the termination resistor driving the noninverting

input.

The signal generator has a symmetric, ground-referenced bipolar

output. The VOCM pin of the ADA4938-1/ADA4938-2 is biased

with an external resistor divider to obtain the desired 3.5 V output

common-mode. One-half of the common-mode voltage is fed

back to the summing nodes, biasing −IN and +IN at 1.75 V. For a

common-mode voltage of 3.5 V, each ADA4938-1/ADA4938-2

output swings between 2.7 V and 4.3 V, providing a 3.2 V p-p

differential output.

The output of the amplifier is dc-coupled to the ADC through a

second-order, low-pass filter with a −3 dB frequency of 50 MHz.

The filter reduces the noise bandwidth of the amplifier and

isolates the driver outputs from the ADC inputs.

The AD9446 is configured for a 4.0 V p-p full-scale input by

setting R1 = R2 = 1 kΩ between the VREF pin and SENSE pin

in Figure 69.

The circuit in Figure 70 shows a simplified front-end connection

for an ADA4938-1/ADA4938-2 driving an AD9246, 14-bit,

125 MSPS ADC. The AD9246 achieves its optimum

performance when it is driven differentially. The ADA4938-1/

ADA4938-2 eliminate the need for a transformer to drive the

ADC, performs a single-ended-to-differential conversion,

buffers the driving signal, and provides appropriate level

shifting for dc coupling.

The ADA4938-1/ADA4938-2 are configured with dual ±5 V

supplies and a gain of ~2 V/V for a single-ended input to

differential output. The 76.8 Ω termination resistor, in parallel

with the single-ended input impedance of 137 Ω, provides a 50 Ω

dc termination for the source. The additional 30.1 Ω (120 Ω total)

at the inverting input balances the parallel dc impedance of the

50 Ω source and the termination resistor driving the

noninverting input.

The signal generator has a symmetric, ground-referenced

bipolar output. The VOCM pin of the ADA4938-1/ADA4938-2 is

connected to the CML pin of the AD9246 to set the output

common-mode level at the appropriate point. A portion of this

is fed back to the summing nodes, biasing −IN and +IN at 0.55 V.

For a common-mode voltage of 0.9 V, each ADA4938-1/

ADA4938-2 output swings between 0.4 V and 1.4 V, providing a

2 V p-p differential output.

The output is dc-coupled to a single-pole, low-pass filter. The filter

reduces the noise bandwidth of the amplifier and provides some

level of isolation from the switched capacitor inputs of the ADC.

The AD9246 is set for a 2 V p-p full-scale input by connecting the

SENSE pin to AGND. The inputs of the AD9246 are biased at

1 V by connecting the CML output, as shown in Figure 70.

VIN+

VIN–

47pF

30nH

30nH

24.3Ω

24.3Ω

50Ω

SIGNAL

GENERATOR

226Ω

200Ω

VOCM

10V

ADA4938

+

61.9Ω

200Ω

200Ω

16

BUFFER T/H

ADC

CLOCK/

TIMING

REF

SENSE

AGND

VREF

AD9446

3.3V (A)

AVDD1

5V (A)

AVDD2

3.3V (D)

DRVDD

10V

R1

R2

Figure 69. ADA4938-1/ADA4938-2 Driving an AD9446, 16-Bit, 80 MSPS ADC

1.8V

DRVDD

AVDD

VIN–

VIN+

AD9246

AGND

CML

SENSE

D13 TO

D0

10pF

33Ω

VOCM

33Ω

50Ω

VIN

90Ω

90Ω

76.8Ω

+5V

–5V

ADA4938

+

200Ω

200Ω

30.1Ω

0.1µF

Figure 70. ADA4938-1/ADA4938-2 Driving an AD9246, a 14-Bit, 125 MSPS ADC