LTC6244HVCMS8 datasheet(17/24 Pages) LINER

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LTC6244

6244f

Figure 5b: Output Noise Spectral Density of Figure 5a. The

Simple JFET Bootstrap Improves Noise (and Bandwidth)

Drastically. Noise Density at 10kHz is Now 220nV/√Hz, About

a 8.2x Reduction. This is Mostly Due to the Bootstrap Effect

of Swapping the 1nV/√Hz of the JFET for the 7.8nV/√Hz of the

Op Amp

VOUT = 1M • IPD

BW = 350kHz

OUTPUT NOISE = 220nV/

√Hz

AT10kHz

IPD

PHILIPS

BF862

JFET

RBIAS

4.99k

HAMAMATSU

LARGE AREA

PHOTODIODE

S1227-1010BQ

CPD = 3000pF

0.25pF

–5V

6244 F04a

–

1/2

LTC6244HV

VOUT

Figure 5a. Large Area Diode Bootstrapping

APPLICATIO S I FOR ATIO

Figure 6b: Output Spectrum of Circuit of Figure 6a, with

Photodiode Bias at 0V. Photodiode Capacitance is Back Up,

as in the Original Circuit of Figure 4a. However, it can be

Reduced Arbitrarily by Providing Reverse Bias. This Plot

Shows that Bootstrapping Alone Reduced the 10kHz Noise

Density by a Factor of 6.2, from 1800nV/√Hz to 291nV/√Hz.

Figure 6a. The Addition of a Capacitor and Resistor Enable the

Beneﬁt of Bootstrapping While Applying Arbitrary Photodiode

Bias Voltage VBB

VOUT = 1M • IPD

BW = 250kHz

OUTPUT NOISE = 291nV/

√Hz

AT 10kHz

VBB

IPD

PHILIPS

BF862

JFET

HAMAMATSU LARGE AREA

PHOTODIODE

S1227-1010BQ

CPD = 3000pF

0.25pF

–5V 6244 F06a

–

1/2

LTC6244HV

–5V

4.99k

4.7

µF

X7R

4.99k

VOUT

by providing reverse bias, and the photodiode can also be

reversed to support either cathode or anode connections

for positive or negative going outputs.

The circuit on the last page of this data sheet shows fur-

ther reduction in noise by paralleling four JFETs to attain

152nV/√Hz at 10kHz, a noise of 12 times less than the

basic photodiode circuit of Figure 4a.

The solution is as shown in the circuit of Figure 6a, which

uses a capacitor-resistor pair to enable the AC beneﬁts of

bootstrapping while allowing a different reverse DC voltage

on the photodiode. The JFET is still running at the same

current, but an arbitrary reverse bias may be applied to

the photodiode. The output noise spectrum of the circuit

with 0V of photodiode reverse bias is shown in Figure 6b.

Photodiode capacitance is again 3650pF, as in the original

circuit of Figure 4a. This noise plot with 0V bias shows

that bootstrapping alone was responsible for a factor of

6.2 noise reduction, from 1800nV/√Hz to 291nV/√Hz at

10kHz, independent of photodiode capacitance. However,

photodiode capacitance can now can be reduced arbitrarily

10k

FREQUENCY (Hz)

6244 F05b

100k

10k

FREQUENCY (Hz)

6244 F06b

100k

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