AD8337BCPZ-R2 datasheet(20/28 Pages) AD

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AD8337

Data Sheet

Rev. D | Page 20 of 28

APPLICATIONS INFORMATION

PREAMPLIFIER CONNECTIONS

Noninverting Gain Configuration

The AD8337 preamplifier is an uncommitted current feedback

op amp that is stable for values of RFB2 ≥ 100 Ω. See Figure 66

for the noninverting feedback connections.

PRAO

PREAMPLIFIER

–

INPP

INPN

RFB2

RFB1

Figure 66. AD8337 Preamplifier Configured for Noninverting Gain

Two surface-mount resistors establish the preamplifier gain.

Equal values of 100 Ω configure the preamplifier for a 6 dB gain

and the device for a default gain range of 0 dB to 24 dB.

For preamplifier gains ≥2, select a value of RFB2 ≥ 100 Ω and

RFB1 ≤ 100 Ω. Higher values of RFB2 reduce the bandwidth and

increase the offset voltage, but smaller values compromise

stability. If RFB1 ≤ 100 Ω, the gain increases and the input

referred noise decreases.

Inverting Gain Configuration

For applications requiring polarity inversion of negative pulses, or

for waveforms that require current sinking, the preamplifier can

be configured as an inverting gain amplifier. When configured

with bipolar supplies, the preamplifier amplifies positive or

negative input voltages with no level shifting of the common-

mode input voltage required. Figure 67 shows the AD8337

configured for inverting gain operation.

Because the AD8337 is a very high frequency device, stability

issues can occur unless the circuit board on which it is used is

carefully laid out. The stability of the preamp is affected by

parasitic capacitance around the INPN pin. To minimize stray

capacitance position the preamp gain resistors, RFB1 and RFB2, as

close as possible to the INPN pin.

PRAO

PREAMPLIFIER

–

INPP

INPN

RFB2

RFB1

Figure 67. The AD8337 Preamplifier Configured for Inverting Gain

DRIVING CAPACITIVE LOADS

Because of the large bandwidth of the AD8337, stray capacitance at

the output pin can induce peaking in the frequency response as the

gain of the amplifier begins to roll off. Figure 68 shows peaking

with two values of load capacitance using ±2.5 V supplies and

VGAIN = 0 V.

–5

100k

FREQUENCY (Hz)

500M

100M

10M

CL = 0pF

CL = 10pF

CL = 22pF

VGAIN = 0V

NO SNUBBING RESISTOR

Figure 68. Peaking in the Frequency Response for Two Values of Output

Capacitance with ±2.5 V Supplies and No Snubbing Resistor

–5

100k

FREQUENCY (Hz)

500M

100M

10M

CL = 0pF

CL = 10pF

CL = 22pF

VGAIN = 0V

WITH 20Ω SNUBBING RESISTOR

Figure 69. Frequency Response for Two Values of Output Capacitance

with a 20 Ω Snubbing Resistor

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

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

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

function of the slewing of the transient (not shown in Figure 70

and Figure 72). The transition time of the input pulses used for

Figure 70 and Figure 72 is deliberately set high at 300 ps to demon-

strate the fast response time of the amplifier. Signals with longer

transition times generate less overshoot.

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