THS4631_14 Datasheet, PDF (11/35 Pages) Texas Instruments – HIGH-VOLTAGE, HIGH SLEW RATE, WIDEBAND FET-INPUT OPERATIONAL AMPLIFIER

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Table 1. Transimpedance Performance Summary
for Various Configurations (continued)
10-kΩ TRANSIMPEDANCE RESPONSES
85
C_S = 18 PF
C_F = 2 PF
80
C_S = 47 PF
C_F = 2.2 PF
75
C_S = 100 PF
C_F = 3 PF
70
V_S = ±15 V
R_L = 1 k
R_F = 10 k
65
10 k
100 k
1M
10 M
1G
f − Frequency − Hz
Figure 35.
100-kΩ TRANSIMPEDANCE RESPONSES
105
C_S = 18 PF
C_F = 0.5 PF
100
C_S = 47 PF
95
C_F = 0.7 PF
C_S = 100 PF
C_F = 1 PF
90
V_S = ±15 V
R_L = 1 k
R_F = 100 k
85
10 k
100 k
1M
10 M
1G
f − Frequency − Hz
Figure 36.
1-MΩ TRANSIMPEDANCE RESPONSES
125
C_S = 18 PF
C_F = 0 PF
120
115
C_S = 47 PF
C_F = 0.2 PF
110
C_S = 100 PF
105
C_F = 0.2 PF
100
V_S = ±15 V
R_L = 1 k
R_F = 1 M
95
10 k
100 k
1M
f − Frequency − Hz
Figure 37.
10 M
MEASURING TRANSIMPEDANCE
BANDWIDTH
While there is no substitute for measuring the
performance of a particular circuit under the exact
conditions that are used in the application, the
complete system environment often makes
measurements harder. For transimpedance circuits, it
Copyright © 2004–2011, Texas Instruments Incorporated
THS4631
SLOS451B – DECEMBER 2004 – REVISED AUGUST 2011
is difficult to measure the frequency response with
traditional laboratory equipment because the circuit
requires a current as an input rather than a voltage.
Also, the capacitance of the current source has a
direct effect on the frequency response. A simple
interface circuit can be used to emulate a capacitive
current source with a network analyzer. With this
circuit, trans- impedance bandwidth measurements
are simplified, making amplifier evaluation easier and
faster.
Network Analizer
I_O
50 W 50 W
C2
R_S
C1
V_S
ǒ Ǔ ^IO
V_S
(s)
+
2 R_S
1
1
)
C1
C2
(Above the Pole Frequency)
A. The interface network creates a capacitive,
constant current source from a network
analyzer and properly terminates the
network analyzer at high frequencies.
Figure 38. Emulating a Capacitive Current Source
With a Network Analyzer
The transconductance transfer function of the
interface circuit is:
s
ǒ Ǔ _IO
V_S
(s)
+
s
2 R_S
)₂
1)^CC12
1
R_S ǒC1)C2Ǔ
(3)
The transfer function contains a zero at dc and a pole
1
at: ^{2 R}S ^{(C1 ) C2)} . The transconductance is constant
1
ǒ Ǔ _at:
2 R_S
1
)
C1
C2
,
above
the
pole
frequency,
providing a controllable ac-current source. This circuit
also properly terminates the network analyzer with 50
Ω at high frequencies. The second requirement for
this current source is to provide the desired output
impedance, emulating the output impedance of a
photodiode or other current source. The output
impedance of this circuit is given by:
ȧȱȲ _{ǒ Ǔ} ȧȳȴ ^Z_O^(s)
+
C1
C1
)
C2
C2
s
)
2
R_S
1
ǒC1)C2Ǔ
s
s
)
2
1
R_S
C1
(4)
Assuming C1 >> C2, the equation reduces to:
Z_O
[
1
s C2
,
giving
the
appearance
of
a
capacitive
source at a higher frequency.
Capacitor values should be chosen to satisfy two
requirements. First, C2 represents the anticipated
capacitance of the true source. Second C1 is chosen
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