AD8337BCPZ-R2 datasheet(21/28 Pages) AD

21 / 28 page

Data Sheet

AD8337

Rev. D | Page 21 of 28

–600

–200

600

400

–20

–400

TIME (ns)

–10

–60

–20

–40

200

OUTPUT

800

–800

–80

INPUT

CL = 0pF

CL = 10pF

CL = 22pF

NO SNUBBING RESISTOR

Figure 70. Pulse Response for Two Values of Output Capacitance

with ±2.5 V Supplies and No Snubbing Resistor

CL = 0pF

CL = 10pF

CL = 22pF

WITH 20Ω SNUBBING RESISTOR

–600

–200

600

400

–20

–400

TIME (ns)

–10

–60

–20

–40

200

INPUT

OUTPUT

800

–800

–80

Figure 71. Pulse Response for Two Values of Output Capacitance

with ±2.5 V Supplies and a 20 Ω Snubbing Resistor

CL = 0pF

CL= 10pF

CL = 22pF

WITH NO SNUBBING RESISTOR

–600

–200

600

400

–20

–400

TIME (ns)

–10

–60

–20

–40

200

INPUT

OUTPUT

800

–800

–80

VS = ±5V

Figure 72. Large Signal Pulse Response for Two Values of Output

Capacitance with ±5 V Supplies and No Snubbing Resistor

CL = 0pF

CL = 10pF

CL = 22pF

WITH 20Ω SNUBBING RESISTOR

–600

–200

600

400

–20

–400

TIME (ns)

–10

–60

–20

–40

200

INPUT

OUTPUT

800

–800

–80

VS = ±5V

Figure 73. Pulse Response for Two Values of Output Capacitance

with ±5 V Supplies and a 20 Ω Snubbing Resistor

The effects of stray output capacitance are mitigated with a

small value snubbing resistor, RSNUB, placed in series with, and

as near as possible to, the VOUT pin. Figure 69, Figure 71, and

Figure 73 show the improvement in dynamic performance with

a 20 Ω snubbing resistor. RSNUB reduces the gain slightly by the

ratio of RL/(RSNUB + RL), a very small loss when used with high

impedance loads, such as ADCs. For other loads, alternate values

of RSNUB can be determined empirically. The data for the curves in

the Typical Performance Characteristics section are derived using

a 20 Ω snubbing resistor.

The best way to avoid the effects of stray capacitance is to

exercise care in the PCB layout. Locate the passive components

or devices connected to the AD8337 output pins as close as

possible to the package.

Although a nonissue, the preamplifier output is also sensitive to

load capacitance. However, the series connection of RFB1 and

RFB2 is typically the only load connected to the preamplifier. If

overshoot appears, it can be mitigated by inserting a snubbing

resistor, the same way as the VGA output.

GAIN CONTROL CONSIDERATIONS

In typical applications, voltages applied to the GAIN input are dc

or relatively low frequency signals. The high input impedance of

the AD8337 enables several devices to be connected in parallel.

This is useful for arrays of VGAs, such as those used for calibra-

tion adjustments.

Under dc or slowly changing ramp conditions, the gain tracks

the gain control voltage, as shown in Figure 3. However, it is often

necessary to consider other effects influenced by the VGAIN input.

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