OPA2381 Datasheet, PDF (11/19 Pages) Burr-Brown (TI) – Precision, Low Power, 18MHz Transimpedance Amplifier

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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_{RDIODE ) RF}Ǔ
2pR_DIODER_F^ǒC_TOT ) C_F^Ǔ
(3)
OPA381
OPA2381
SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004
(a)
RF = 50kΩ
C_STRAY = 0.2pF
λ
OPA381
V_BIAS
V_OUT
(b)
RF = 50kΩ
C_STRAY = 0.2pF
C_F = 16pF
λ
OPA381
V_OUT
V_BIAS
(c)
RF = 50kΩ
C_STRAY = 0.2pF
CF = 22pF
RFILTER
λ
= 102kΩ
OPA381
V_OUT
CFILTER
= 3.9nF
V_BIAS
Figure 6. Transimpedance Circuit Configurations
with Varying Total and Integrated Noise Gain
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