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2.2 THE DETECTOR

The detector used in the SSP-4 is a model G5851 manufactured by Hamamatsu Corporation.

This unit can be classified as an InGaAs-PIN photodiode, which offers the highest possible shunt

resistance value (R

) resulting in a very low noise current when used in the photovoltaic mode.

See Appendix A for complete description of this detector.

The detector is normally operated in a cooled state down to –40 C using a two stage

thermoelectrically Peltier cooler. This decreases thermal noise from the detector and increases

shunt resistance, which reduces the noise voltage contribution from the preamp from swamping

the total noise budget. At room temperature the shunt resistance is under 1M and over 200M

at -40 C. The operating temperature is made stable by a feedback system using an integrated

thermistor mounted within the detector package.

Size

TO-8 can

Active Diameter

0.30 mm, 1.00mm optional

Spectral Range

900 to 2050 nm

Peak Response

1.2 A/W at 1950 nm

Normal Operating Temp.

-35 C

Shunt Resistance

>0.2 G at -35 C

Noise Equivalent Power

2x10

-14

at -35 C

Table 2-1. Physical and Electrical Characteristics of the Model G5851-203

2.3 DETECTOR/ELECTROMETER DESIGN

The detector/electrometer circuit is the heart of the SSP-4 photometer and its design is a matter

of optimizing both key elements as a single working unit.

Figure 2-2 shows the basic current-to-voltage amplifier configuration used in the SSP-4.

Photocurrent from the detector is balanced by an equal current in the feedback resistor, but

flowing in the opposite direction so that the inverting input is kept near zero potential. The output

voltage is thus:

out

= - R

x I

where R

is the feedback resistance and, in the case of the SSP-4, is equal to 1 G ohm (1 x 10

The photocurrent from the photodiode is I

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