AD797 datasheet(13/20 Pages) AD

13 / 20 page

AD797

Rev. F | Page 13 of 20

USE SHORT

LEAD LENGTHS

(<5mm)

KELVIN RETURN

LOAD

CURRENT

0.1µF

4.7µF

USE SHORT

LEAD LENGTHS

(<5mm)

KELVIN RETURN

LOAD

CURRENT

0.1µF

4.7µF TO 22.0µF

1.1Ω TO 4.7Ω

Figure 35. 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 (Figure 36) 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.

100Ω

VOUT

VIN

+VS

–VS

AD797

600Ω

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 36. 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 37). The input source resistance and capacitance also

affect the response slightly, and experimentation may be

necessary for best results.

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

VOUT

VIN

+VS

–VS

AD797

600Ω

100Ω

Figure 37. Alternative Voltage Follower Connection

Low noise preamplification is usually performed in the non-

inverting mode (Figure 38). 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.

VOUT

VIN

+VS

–VS

AD797

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 38. Low Noise Preamplifier

Table 4 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 50) 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 38). Table 4 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 4. Values for Follower with Gain Circuit

Gain

Noise

(Excluding RS)

1 kΩ

≈20 pF

3.0 nV/√Hz

300 Ω

≈10 pF

1.8 nV/√Hz

33.2 Ω

300 Ω

≈5 pF

1.2 nV/√Hz

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 configu-

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

example as a DAC buffer, the circuit shown in Figure 39 should

be used. The value of CL depends on the DAC, and if CL is greater

than 33 pF, a 100 Ω series resistor is required. A bypassed balancing

resistor (RS and CS) can be included to minimize dc errors.

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