AD8337BCPZ-R2 datasheet(21/24 Pages) AD

21 / 24 page

AD8337

Rev. B | Page 21 of 24

In the time domain, stray capacitance at the output pin can

induce overshoot on the edges of transient signals, as seen in

Figure 70 and Figure 72. The amplitude of the overshoot is

also a function of the slewing of the transient (not shown).

The transition time of the input pulses used for Figure 70

and Figure 72 was set deliberately high at 300 ps to demonstrate

the fast response time of the amplifier. Signals with longer

transition times generate less overshoot.

–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 20

Ω 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 output pin.

, and

show the improvement in dynamic performance with

a 20 Ω snubbing resistor. R

Figure 69 Figure 71

Figure 73

SNUB

reduces the gain slightly by the

ratio of RLOAD/(RSNUB + RLOAD), 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 of this

data sheet are derived using a 20 Ω snubbing resistor.

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

exercise care in PC board 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 in the same way as the

VGA output, by inserting a snubbing resistor.

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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