AD8337BCPZ-R2 datasheet(20/24 Pages) AD

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AD8337

Rev. B | Page 20 of 24

APPLICATIONS

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 preamp 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. Position the pre-

amp gain resistors, RFB1 and RFB2, as close as possible to Pin 4,

INPN, to minimize stray capacitance.

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

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