LT6204 Linear Technology, LT6204 Datasheet - Page 17

• Manufacturer Part Number: LT6204
• Description: (LT6202 - LT6204) Low Power Op Amps
• Manufacturer: Linear Technology

Low Noise, Low Power 1M AC
Photodiode Transimpedance Amplifier
Figure 3 shows the LT6202 applied as a transimpedance
amplifier (TIA). The LT6202 forces the BF862 ultralow-
noise JFET source to 0V, with R3 ensuring that the JFET
has an I
buffering the input of the LT6202 and making it suitable for
the high impedance feedback elements R1 and R2. The
BF862 has a minimum I
between –0.3V and –1.2V. The JFET gate and the LT6202
TYPICAL APPLICATIO S
V
DRAIN
BIAS
Figure 3. Low Noise, Low Power 1M
AC Photodiode Transimpedance Amplifier
–
4.99k
of 1mA. The JFET acts as a source follower,
R3
V
V
S
S
+
–
PHILIPS
BF862
V
BIAS
DSS
–
of 10mA and a pinchoff voltage
499k
R1
–
+
Figure 4. Precision Low Noise, Low Power Transimpedance Amplifier
LT6202
V
R4
10M
S
= 5V
U
–
+
LTC2050HV
499k
1pF
C1
R2
0.1 F
C2
LT6202/03/04 F03
V
OUT
10k
R5
1 F
C3
output therefore sit at a point slightly higher than one
pinchoff voltage below ground (typically about –0.6V).
When the photodiode is illuminated, the current must
come from the LT6202’s output through R1 and R2, as in
a normal TIA. Amplifier input noise density and gain-
bandwidth product were measured at 2.4nV/Hz and
100MHz, respectively. Note that because the JFET has a
high g
R3 is only about 2%. Gain-bandwidth product was mea-
sured at 100MHz and the closed-loop bandwidth using a
3pF photodiode was approximately 1.4MHz.
Precision Low Noise, Low Power, 1M
Photodiode Transimpedance Amplifier
Figure 4 shows the LT6202 applied as a transimpedance
amplifier (TIA), very similar to that shown in Figure 3. In
this case, however, the JFET is not allowed to dictate the
DC-bias conditions. Rather than being grounded, the
LT6202’s noninverting input is driven by the LTC2050 to
the exact state necessary for zero JFET gate voltage. The
noise performance is nearly identical to that of the circuit
in Figure 3, with the additional benefit of excellent DC
performance. Input offset was measured at under 200 V
and output noise was within 2mV
bandwidth.
4.99k
R3
V
V
S
S
m
–
+
PHILIPS
BF862
, approximately 1/80 , its attenuation looking into
LT6202/LT6203/LT6204
499k
–
+
R1
LT6202
V
S
= 5V
499k
1pF
C1
R2
LT6202/03/04 F04
V
OUT
P-P
over a 20MHz
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