Datasheet

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SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004
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11
TRANSIMPEDANCE BANDWIDTH AND
NOISE
Limiting the gain set by R
F
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 R
F
increases with the square-root of R
F
,
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, C
F
, across the feedback resistor, R
F
, 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 a 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 (R
FILTER
and C
FILTER
) 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 R
DIODE
to represent the equivalent diode
resistance, and C
TOT
for equivalent diode capacitance
plus OPA381 input capacitance, the noise zero, f
Z
, is
calculated by:
f
Z
+
ǒ
R
DIODE
) R
F
Ǔ
2pR
DIODE
R
F
ǒ
C
TOT
) C
F
Ǔ
C
FILTER
= 3.9nF
R
FILTER
= 102kΩ
C
F
= 22pF
OPA381
R
F
= 50kΩ
OPA381
OPA381
R
F
= 50kΩ
R
F
= 50kΩ
C
F
= 16pF
V
OUT
V
BIAS
(b)
λ
C
STRAY
= 0.2pF
V
OUT
V
BIAS
(a)
λ
C
STRAY
=0.2pF
V
OUT
V
BIAS
(c)
λ
C
STRAY
=0.2pF
Figure 6. Transimpedance Circuit Configurations
with Varying Total and Integrated Noise Gain
(3)