ADA4945-1ACPZ-R7 datasheet(53/57 Pages) AD

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Data Sheet

ADA4945-1

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Rev. A

53 of 57

Figure 114. Clamped Differential Signal Levels

Figure 115. Clamped Common-Mode Signal Levels

Driving a High Precision ADC

The ADA4945-1 is ideally suited for broadband dc-coupled applications. The recommended list of precision

converters is shown in Table 15. The circuit in Figure 116 shows an example of the ADA4945-1 driving a precision ADC

such as the AD4003 (an 18-bit, 2 MSPS, successive approximation ADC), or the AD7768 (a 24-bit, 256 kSPS, sigma-

delta ADC). The ADA4945-1 is dc-coupled on the input and the output, which eliminates the need for a transformer

to drive the ADC. In this example, the ADA4945-1 is applied in a differential input to differential output configuration,

with a gain of 1, and with dual supplies of +7 V and −2 V. The output of the ADA4945-1 is level shifted to match the

input common mode of the ADC. The gain is set by the ratio of the feedback resistor to the gain resistor. In addition,

the circuit can be used in a single-ended input to differential output configuration. If needed, a termination resistor

in parallel with the source input can be used. When a single-ended input is used, the input impedance of the amplifier

can be calculated as shown in the Terminating a Single-Ended Input section. If the feedback and gain resistors are

all 1 kΩ, as in Figure 116, the single-ended input impedance is approximately 1.33 kΩ, which, in parallel with a

52.3 Ω termination resistor, provides a 50 Ω termination for the source. An additional 25.5 Ω (1025.5 Ω total) at the

inverting input balances the parallel impedance of the 50 Ω source and the termination resistor driving the

noninverting input. However, if a differential source input is used, the differential input impedance is 2 kΩ. In this

case, two 52.3 Ω termination resistors are used to terminate the inputs.

When driving the AD7768 in this example, the ADA4945-1 is driven by a signal generator having an 8 V p-p symmetric,

bipolar output. The VOCM input of the ADA4945-1 is bypassed for noise reduction and is driven via the common-mode

TIME (µs)

+VS = +7V, –VS = –2V

+VCLAMP = +5V, –VCLAMP = 0V

VOCM = +2.5V

+DIN

–DIN

+OUT

–OUT

TIME (µs)

+VS = +7V, –VS = –2V

+VCLAMP = +5V, –VCLAMP = 0V

+DIN = –DIN = 0V

VOCM

+OUT, –OUT

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