Datasheet

"#$
%"#$
SBOS291G − NOVEMBER 2003 − REVISED SEPTEMBER 2007
www.ti.com
12
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 10MHz. Adding a 16pF feedback
capacitor (Figure 6b) will limit the bandwidth and result
in a –3dB point at approximately 1MHz (see 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 OPA380 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
OPA380
V
OUT
V
BIAS
R
F
=10k
Ω
(a)
λ
C
STRAY
=0.2pF
C
F
=16pF
OPA380
V
OUT
V
BIAS
R
F
= 10k
Ω
(b)
λ
C
STRAY
= 0.2pF
V
OUT
C
FILTER
= 796pF
R
FILTER
= 100
Ω
C
F
= 21pF
OPA380
V
BIAS
R
F
=10k
Ω
(c)
λ
C
STRAY
=0.2pF
Figure 6. Transimpedance Circuit Configurations
with Varying Total and Integrated Noise Gain
(3)