AD797ARZ datasheet(14/20 Pages) AD

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AD797

Rev. F | Page 14 of 20

VOUT

+VS

–VS

AD797

IIN

100Ω

600Ω

20pF TO 120pF

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 39. I-to-V Converter Connection

THE INVERTING CONFIGURATION

The inverting configuration (see Figure 40) presents a low input

impedance, R1, to the source. For this reason, the goals of both

low noise and input buffering are at odds with one another.

Nonetheless, the excellent dynamics of the AD797 makes

it the preferred choice in many inverting applications, and

with careful selection of feedback resistors, the noise penalties

are minimal. Some examples are presented in Table 5 and

Figure 40.

VOUT

VIN

+VS

–VS

AD797

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 40. Inverting Amplifier Connection

Table 5. Values for Inverting Circuit

Gain

Noise

(Excluding RS)

−1

1 kΩ

≈20 pF

3.0 nV/√Hz

−1

300 Ω

≈10 pF

1.8 nV/√Hz

−10

150 Ω

1500 Ω

≈5 pF

1.8 nV/√Hz

DRIVING CAPACITIVE LOADS

The capacitive load driving capabilities of the AD797 are

displayed in Figure 41. At gains greater than 10, usually no

special precautions are necessary. If more drive is desirable,

however, the circuit shown in Figure 42 should be used. For

example, this circuit allows a 5000 pF load to be driven cleanly

at a noise gain ≥2.

100nF

10nF

1pF

100

1nF

100pF

10pF

CLOSED-LOOP GAIN

Figure 41. Capacitive Load Drive Capability vs. Closed-Loop Gain

VOUT

VIN

+VS

–VS

AD797

20pF

200pF

33Ω

100Ω

1kΩ

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 42. Recommended Circuit for Driving a High Capacitance Load

SETTLING TIME

The AD797 is unique among ultralow noise amplifiers in that it

settles to 16 bits (<150 μV) in less than 800 ns. Measuring this

performance presents a challenge. A special test circuit (see

Figure 43) was developed for this purpose. The input signal was

obtained from a resonant reed switch pulse generator, available

from Tektronix as calibration Fixture No. 067-0608-00. When

open, the switch is simply 50 Ω to ground and settling is purely

a passive pulse decay and inherently flat. The low repetition rate

signal was captured on a digital oscilloscope after being

amplified and clamped twice. The selection of plug-in for the

oscilloscope was made for minimum overload recovery.

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