LT6202 Datasheet, PDF (17/24 Pages) Linear Technology – Rail-to-Rail Input and Output, Ultralow 1.9n Root Hz Noise, Low Power Op Amps

LT6202/LT6203/LT6204
TYPICAL APPLICATIO S
Low Noise, Low Power 1MΩ 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_DRAIN of 1mA. The JFET acts as a source follower,
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_DSS of 10mA and a pinchoff voltage
between –0.3V and –1.2V. The JFET gate and the LT6202
V_S+
R1
R2
499k
499k
V_BIAS–
PHILIPS
BF862
C1
–
1pF
LT6202
+
V_OUT
R3
4.99k
V_S = ±5V
V_S–
LT6202/03/04 F03
Figure 3. Low Noise, Low Power 1MΩ
AC Photodiode Transimpedance Amplifier
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_m, approximately 1/80Ω, its attenuation looking into
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, 1MΩ
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 200µV
and output noise was within 2mV_P-P over a 20MHz
bandwidth.
V_S+
R1
R2
499k
499k
V_BIAS–
R4
C2
10M
0.1µF
–
LTC2050HV
+
R5
10k
C3
1µF
PHILIPS
BF862
C1
1pF
–
LT6202
+
R3
4.99k
V_OUT
V_S–
V_S = ±5V
LT6202/03/04 F04
Figure 4. Precision Low Noise, Low Power Transimpedance Amplifier
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