OPA659IDBVT datasheet(13/25 Pages) TI

13 / 25 page

Wideband, Inverting Gain Operation

Wideband, High-Sensitivity, Transimpedance

OPA659

OUT

0.1 F

10 F

0.1 F

10 F

Load

Source

OUT

+6V

-6V

OPA659

OUT

0.1 F

10 F

0.1 F

10 F

Load

OUT

Photo

Diode

-V

+6V

-6V

OPA659

www.ti.com........................................................................................................................................................................................... SBOS342 – DECEMBER 2008

Figure 36 shows the noninverting input tied directly to

ground. Often, a bias current-cancelling resistor to

The circuit of Figure 36 shows the inverting gain test

ground is included here to nullify the dc errors caused

circuit

used

for

most

the

inverting

Typical

by input bias current effects. For a JFET input op

Characteristics graphs. As with the noninverting

amp such as the OPA659, the input bias currents are

applications, most of the curves were characterized

so low that dc errors caused by input bias currents

using signal sources with 50

Ω driving impedance,

are

negligible.

Thus,

bias

current-cancelling

and with measurement equipment that presents a

resistor is recommended at the noninverting input.

Ω load impedance. In Figure 36, the shunt resistor

RT at VIN should be set so the parallel combination of

the shunt resistor and RG equals 50Ω to match the

Design

source impedance of the test generator and cable,

while the series output resistor ROUT at VOUT should

The high GBP and low input voltage and current

also be set to 50

Ω to provide matching impedance for

noise for the OPA659 make it an ideal wideband,

the

measurement

equipment

load

and

cable.

transimpedance

amplifier

for

low

moderate

Generally, data sheet voltage swing specifications are

transimpedance gains. Higher transimpedance gains

measured at the output pin, VOUT, in Figure 36.

(above 100k

Ω) can benefit from the low input noise

current of a JFET input op amp such as the OPA659.

Designs that require high bandwidth from a large

area detector can benefit from the low input voltage

noise for the OPA659. This input voltage noise is

peaked up over frequency by the diode source

capacitance, and in many cases, may become the

limiting factor to input sensitivity. The key elements to

the design are the expected diode capacitance (CD)

with the reverse bias voltage (–VB) applied, the

desired transimpedance gain, RF, and the GBP for

the OPA659 (350MHz). Figure 37 shows a general

transimpedance amplifier circuit, or TIA, using the

OPA659. Given the source diode capacitance plus

parasitic input capacitance for the OPA659, the

transimpedance gain, and known GBP, the feedback

Figure 36. General Inverting Test Circuit

capacitor value, CF, may be calculated to avoid

excessive peaking in the frequency response.

The inverting circuit can also use a wide range of

resistor values to set the gain; Table 2 lists several

recommended resistor values for inverting gains with

a 50

Ω input/output match.

Table 2. Resistor Values for Inverting Gains with

Ω Input/Output Match

INVERTING

GAIN

ROUT

–1

249

61.9

49.9

–2

249

124

84.5

49.9

–5

249

49.9

Open

49.9

–10

499

49.9

Open

49.9

Figure 37. Wideband, Low-Noise, Transimpedance

Amplifier (TIA)

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