AD797

REV. C

–11–

600

100

20–120pF

AD797

**

2

7

3

4

6

**

* SEE TEXT

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

R1

Ω

Ω

Figure 36. I-to-V Converter Connection

greater than 33 pF a 100

Ω series resistor is required. A by-

mize dc errors.

THE INVERTING CONFIGURATION

The inverting configuration (Figure 37) 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 will make it

the preferred choice in many inverting applications, and with care-

ful selection of feedback resistors the noise penalties will be mini-

mal. Some examples are presented in Table II and Figure 37.

AD797

**

2

7

3

4

6

**

* SEE TEXT

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

R2

R1

Figure 37. Inverting Amplifier Connection

Table III. Values for Inverting Circuit

Noise

Gain

R1

R2

–1

1 k

Ω

1 k

Ω≈20 pF

3.0 nV/

√Hz

–1

300

Ω

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 dis-

played in Figure 38. At gains over 10 usually no special precau-

tions are necessary. If more drive is desirable the circuit in

Figure 39 should be used. Here a 5000 pF load can be driven

cleanly at any noise gain

100nF

10nF

1pF

110

1k

100

1nF

100pF

10pF

CLOSED-LOOP GAIN

Figure 38. Capacitive Load Drive Capability vs. Closed

Loop Gain

100

1k

C1

1k

20pF

33

AD797

**

2

7

3

4

6

**

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

200pF

Ω

Ω

Ω

Ω

Figure 39. 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 setup (Figure

40) was developed for this purpose. The input signal was ob-

tained 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 ampli-

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

scope was made for minimum overload recovery.