LTC6244HVCMS8 datasheet(16/24 Pages) LINER

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LTC6244

6244f

APPLICATIO S I FOR ATIO

Large Area Photodiode Ampliﬁers

A simple large area photodiode ampliﬁer is shown in

Figure 4a. The capacitance of the photodiode is 3650pF

(nominally 3000pF), and this has a signiﬁcant effect on

the noise performance of the circuit. For example, the

photodiode capacitance at 10kHz equates to an impedance

of 4.36k

Ω, so the op amp circuit with 1MΩ feedback has a

noise gain of NG = 1 + 1M/4.36k = 230 at that frequency.

Therefore, the LTC6244 input voltage noise gets to the

output as NG • 7.8nV/√Hz = 1800nV/√Hz, and this can

clearly be seen in the circuit’s output noise spectrum in

Figure 4b. Note that we have not yet accounted for the

op amp current noise, or for the 130nV/√Hz of the gain

resistor, but these are obviously trivial compared to the op

amp voltage noise and the noise gain. For reference, the

DC output offset of this circuit is about 100µV, bandwidth

is 52kHz, and the total noise was measured at 1.7mVRMS

on a 100kHz measurement bandwidth.

An improvement to this circuit is shown in Figure 5a,

where the large diode capacitance is bootstrapped by a

1nV/√Hz JFET. This depletion JFET has a VGS of about

–0.5V, so that RBIAS forces it to operate at just over 1mA of

drain current. Connected as shown, the photodiode has a

reverse bias of one VGS, so its capacitance will be slightly

lower than in the previous case (measured 2640pF), but

the most drastic effects are due to the bootstrapping.

Figure 5b shows the output noise of the new circuit.

Noise at 10kHz is now 220nV/√Hz, and the 130nV/√Hz

noise thermal noise ﬂoor of the 1M feedback resistor

is discernible at low frequencies. What has happened is

that the 7.8nV/√Hz of the op amp has been effectively

replaced by the 1nV/√Hz of the JFET. This is because the

1M feedback resistor is no longer “looking back” into the

large photodiode capacitance. It is instead looking back

into a JFET gate capacitance, an op amp input capacitance,

and some parasitics, approximately 10pF total. The large

photodiode capacitance is across the gate-source volt-

age of the low noise JFET. Doing a sample calculation at

10kHz as before, the photodiode capacitance looks like

Ω, so the 1nV/√Hz of the JFET creates a current noise

of 1nV/6k = 167fA/√Hz. This current noise necessarily

ﬂows through the 1M feedback resistor, and so appears

as 167nV/√Hz at the output. Adding the 130nV/√Hz of the

resistor (RMS wise) gives a total calculated noise density

of 210nV/√Hz, agreeing well with the measured noise of

Figure 5b. Another drastic improvement is in bandwidth,

now over 350kHz, as the bootstrap enabled a reduction

of the compensating feedback capacitance. Note that the

bootstrap does not affect the DC accuracy of the ampliﬁer,

except by adding a few picoamps of gate current.

There is one drawback to this circuit. Most photodiode

circuits require the ability to set the amount of applied

reverse bias, whether it’s 0V, 5V, or 200V. This circuit has

a ﬁxed reverse bias of about 0.5V, dictated by the JFET.

Figure 4b. Output Noise Spectral Density of the Circuit of Figure

4a. At 10kHz, the 1800nV/√Hz Output Noise is Due Almost

Entirely to the 7.8nV Voltage Noise of the LTC6244 and the High

Noise Gain of the 1M Feedback Resistor Looking Into the High

Photodiode Capacitance

VOUT = 1M • IPD

BW = 52kHz

NOISE = 1800nV/

√Hz AT 10kHz

IPD

HAMAMATSU

LARGE AREA

PHOTODIODE

S1227-1010BQ

CPD = 3000pF

3.9pF

–5V 6244 F04a

–

1/2

LTC6244HV

VOUT

Figure 4a. Large Area Photodiode Transimpedance Ampliﬁer

10k

FREQUENCY (Hz)

6244 F04b

100k

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