OPA381 datasheet(11/24 Pages) TI1

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OPA381

OPA2381

SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004

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TRANSIMPEDANCE BANDWIDTH AND

NOISE

Limiting the gain set by RF can decrease the noise

occurring at the output of the transimpedance circuit.

However, all required gain should occur in the

transimpedance stage, since adding gain after the

transimpedance amplifier generally produces poorer

noise

performance.

The

noise

spectral

density

produced by RF increases with the square-root of RF,

whereas the signal increases linearly. Therefore,

signal-to-noise ratio is improved when all the required

gain is placed in the transimpedance stage.

Total noise increases with increased bandwidth. Limit

the circuit bandwidth to only that required. Use a

capacitor, CF, across the feedback resistor, RF, to limit

bandwidth (even if not required for stability), if total

output noise is a concern.

Figure 6a shows the transimpedance circuit without any

feedback capacitor. The resulting transimpedance gain

of this circuit is shown in Figure 7. The –3dB point is

approximately

3MHz.

Adding

16pF

feedback

capacitor (Figure 6b) will limit the bandwidth and result

in a –3dB point at approximately 200kHz (seen in

Figure 7). Output noise will be further reduced by

adding a filter (RFILTER and CFILTER) to create a second

pole (Figure 6c). This second pole is placed within the

feedback loop to maintain the amplifier’s low output

impedance. (If the pole was placed outside the

feedback loop, an additional buffer would be required

and would inadvertently increase noise and dc error).

Using RDIODE to represent the equivalent diode

resistance, and CTOT for equivalent diode capacitance

plus OPA381 input capacitance, the noise zero, fZ, is

calculated by:

DIODE ) RF

DIODERF CTOT ) CF

CFILTER

= 3.9nF

RFILTER

= 102k

Ω

CF = 22pF

OPA381

RF = 50kΩ

OPA381

RF = 50kΩ

F = 16pF

OUT

BIAS

(b)

STRAY = 0.2pF

OUT

BIAS

(a)

STRAY =0.2pF

OUT

BIAS

(c)

STRAY =0.2pF

Figure 6. Transimpedance Circuit Configurations

with Varying Total and Integrated Noise Gain

(3)

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