AD797

REV. C

–10–

BYPASSING CONSIDERATIONS

To take 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–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 sup-

ply bypassing. In this case selective use of larger values of tanta-

lum capacitors and damping of their lead inductance with small

value (1.1

Ω to 4.7 Ω) carbon resistors can be an improvement.

Figure 32 summarizes bypassing recommendations. The symbol

(**) is used throughout this data sheet to represent the parallel

combination of a 0.1

µF and a 4.7 µF capacitor.

0.1

µF

USE SHORT

LEAD LENGTHS

(<5mm)

KELVIN RETURN

LOAD

CURRENT

4.7 – 22.0

µF

USE SHORT

LEAD RETURNS

(<5mm)

OR

4.7

µF

1.1 – 4.7

Ω

0.1

µF

KELVIN RETURN

LOAD

CURRENT

Figure 32. Recommended Power Supply Bypassing

THE NONINVERTING CONFIGURATION

parasitic resistance) for the input transistors (

≈6 Ω). This im-

plies very little damping of input and output reactive interac-

tions. With the AD797, additional input series damping is

required for stability with direct input to output feedback. A

100

Ω resistor in the inverting input (Figure 33) is sufficient;

the 100

Ω balancing resistor (R2) is recommended, but is not

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

flatness is obtained with the addition of a small capacitor

Ω resistor (Figure 34).

The input source resistance and capacitance will also affect the

response slightly and experimentation may be necessary for best

results.

R2

100

Ω

R1

100

Ω

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

600

Ω

AD797

**

2

7

3

4

6

**

Figure 33. Voltage Follower Connection

Low noise preamplification is usually done in the noninverting

mode (Figure 35). 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

due consideration to load drive and power consumption. Table

II gives some representative values for the AD797 as a low noise

follower. Operation on 5 volt supplies allows the use of a 100

Ω

or less feedback network (R1 + R2). Since the AD797 shows

no unusual behavior when operating near its maximum rated

current, it is suitable for driving the AD600/AD602 (Figure 47)

while preserving their low noise performance.

Optimum flatness and stability at noise gains >1 sometimes

resistor (R1, Figure 35). Table II includes recommended values

100

Ω and C

Ω resistor should

assumed, and the AD797 should never be operated with unbal-

anced source resistance >200 k

Ω/G.

100

AD797

**

2

7

3

4

6

**

* SEE TEXT

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

600

Ω

Ω

Figure 34. Alternative Voltage Follower Connection

R2

R1

R

L

AD797

**

2

7

3

4

6

**

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

Figure 35. Low Noise Preamplifier

Table II. Values for Follower With Gain Circuit

Noise

Gain

R1

R2

21 k

Ω

1 k

Ω≈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

The I-to-V converter is a special case of the follower configura-

tion. When the AD797 is used in an I-to-V converter, for in-

stance as a DAC buffer, the circuit of Figure 36 should be used.