ADA4870 datasheet(24/25 Pages) AD

24 / 25 page

Data Sheet

ADA4870

Rev. 0 | Page 23 of 24

COMPOSITE AMPLIFIER

When dc precision and high output current are required, the

ADA4870 can be combined with a precision amplifier such as

the ADA4637-1 to form a composite amplifier as shown in

Figure 77.

By placing the ADA4870 inside the feedback loop of the

ADA4637-1, the composite amplifier provides the high output

current of the ADA4870 while preserving the dc precision of

the ADA4637-1.

Figure 77. Composite Amplifier

Figure 78 shows the bandwidth of the composite amplifier at a

gain of 10. The offset voltage at the output is <500 μV.

The circuit can be tailored for different gains as desired.

Depending on the board parasitics, the 6 pF capacitor may need

to be empirically adjusted to optimize performance. Minimize

PCB stray capacitance as much as possible, particularly in the

feedback path.

The small signal and large signal pulse response is shown in

Figure 79 and Figure 80, respectively.

Figure 78. Composite Amplifier Frequency Response

Figure 79. Composite Amplifier Small Signal Pulse Response

Figure 80. Composite Amplifier Large Signal Pulse Response

VIN

VOUT

AV = 10V/V

OUTPUT OFFSET <500µV

50Ω

ADA4637-1

ADA4870

3.01kΩ

1kΩ

6pF

10Ω

3.01kΩ

–35

–30

–25

–20

–15

–10

–5

0.1

FREQUENCY (MHz)

100

1000

CL = 300pF

RS = 10Ω

VOUT = 1V p-p

CL = 300pF

RS = 10Ω

VOUT = 10V p-p

RL = 50Ω

VOUT = 1V p-p

–1.5

–1.0

–0.5

0.5

1.0

1.5

TIME (100ns/DIV)

VS = ±15V

CL = 300pF

RS = 10Ω

–15

–10

–5

TIME (100ns/DIV)

VS = ±15V

CL = 300pF

RS = 10Ω

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