Data Sheet

AD797

Rev. K | Page 13 of 19

BYPASSING CONSIDERATIONS

Taking full advantage of the very wide bandwidth and dynamic

range capabilities of the AD797 requires some precautions.

First, multiple bypassing is recommended in any precision

application. A 1.0 μF to 4.7 μF tantalum in parallel with 0.1 μF

ceramic bypass capacitors are sufficient in most applications.

When driving heavy loads, a larger demand is placed on the

supply bypassing. In this case, selective use of larger values of

tantalum capacitors and damping of their lead inductance with

small-value (1.1 Ω to 4.7 Ω) carbon resistors can achieve an

improvement. Figure 36 summarizes power supply bypassing

recommendations.

USE SHORT

LEAD LENGTHS

(<5mm)

KELVIN RETURN

LOAD

CURRENT

OR

0.1µF

VS

VS

4.7µF

USE SHORT

LEAD LENGTHS

(<5mm)

KELVIN RETURN

LOAD

CURRENT

0.1µF

4.7µF TO 22.0µF

1.1Ω TO 4.7Ω

Figure 36. Recommended Power Supply Bypassing

THE NONINVERTING CONFIGURATION

Ultralow noise requires very low values of the internal parasitic

resistance (rBB) for the input transistors (≈6 Ω). This implies

very little damping of input and output reactive interactions.

With the AD797, additional input series damping is required

for stability with direct output to input feedback. A 100 Ω

resistor (R1) in the inverting input (see Figure 37) is sufficient;

the 100 Ω balancing resistor (R2) is recommended but is not

required for stability. The noise penalty is minimal (eNtotal ≈

2.1 nV/√Hz), which is usually insignificant.

7

4

3

R2

100Ω

R1

100Ω

*

*

VOUT

VIN

+VS

–VS

AD797

RL

600Ω

6

2

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 37. Voltage Follower Connection

Best response flatness is obtained with the addition of a small

capacitor (CL < 33 pF) in parallel with the 100 Ω resistor

(Figure 38). The input source resistance and capacitance also

affect the response slightly, and experimentation may be

necessary for best results.

7

*

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

*

VOUT

VIN

+VS

–VS

AD797

CL

6

2

RS

CS

3

4

600Ω

100Ω

Figure 38. Alternative Voltage Follower Connection

Low noise preamplification is usually performed in the non-

inverting mode (see Figure 39). For lowest noise, the equivalent

resistance of the feedback network should be as low as possible.

The 30 mA minimum drive current of the AD797 makes it easier

to achieve this. The feedback resistors can be made as low as

possible, with consideration to load drive and power consumption.

7

*

*

VOUT

VIN

+VS

–VS

AD797

RL

CL

R2

R1

6

2

3

4

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 39. Low Noise Preamplifier

Table 6 provides some representative values for the AD797 when

used as a low noise follower. Operation on 5 V supplies allows

the use of a 100 Ω or less feedback network (R1 + R2). Because

the AD797 shows no unusual behavior when operating near its

maximum rated current, it is suitable for driving the AD600/

AD602 (see Figure 51) while preserving low noise performance.

Optimum flatness and stability at noise gains >1 sometimes require

a small capacitor (CL) connected across the feedback resistor (R1 of

Figure 39). Table 6 includes recommended values of CL for several

gains. In general, when R2 is greater than 100 Ω and CL is greater

than 33 pF, a 100 Ω resistor should be placed in series with CL.

Source resistance matching is assumed, and the AD797 should not

be operated with unbalanced source resistance >200 kΩ/G.

Table 6. Values for Follower with Gain Circuit

Gain

R1

R2

CL

Noise

(Excluding RS)

2

1 kΩ

1 kΩ

≈ 20 pF

3.0 nV/√Hz

2

300 Ω

300 Ω

≈ 10 pF

1.8 nV/√Hz

10

33.2 Ω

300 Ω

≈ 5 pF

1.2 nV/√Hz

20

16.5 Ω

316 Ω

1.0 nV/√Hz

>35

10 Ω

(G − 1) × 10 Ω

0.98 nV/√Hz