LT6204IGN datasheet(17/24 Pages) LINER

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LT6202/LT6203/LT6204

620234fa

Low Noise, Low Power 1M

Ω 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 IDRAIN 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 IDSS of 10mA and a pinchoff voltage

between –0.3V and –1.2V. The JFET gate and the LT6202

Figure 3. Low Noise, Low Power 1M

Ω

AC Photodiode Transimpedance Amplifier

Figure 4. Precision Low Noise, Low Power 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 gm, 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, 1M

Ω

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

µV

and output noise was within 2mVP-P over a 20MHz

bandwidth.

VBIAS

–

PHILIPS

BF862

499k

1pF

–

VOUT

VS = ±5V

LT6202

4.99k

LT6202/03/04 F03

VBIAS

–

PHILIPS

BF862

499k

1pF

–

VOUT

VS = ±5V

4.99k

0.1

µF

LTC2050HV

10M

10k

µF

LT6202/03/04 F04

LT6202

TYPICAL APPLICATIO S

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