TSX9291 datasheet(24/31 Pages) STMICROELECTRONICS

24 / 31 page

Application information

TSX9291, TSX9292

24/31

DocID024568 Rev 4

4.9

High speed photodiode

The TSX929x series is an excellent choice for current to voltage (I-V) conversions. Due to

the CMOS technology, the input bias currents are extremely low. Moreover, the low noise

and high unity-gain bandwidth of TSX9291 TSX9292 make them particularly suitable for

high-speed photodiode preamplifier applications.

The photodiode is considered as a capacitive current source. The input capacitance, CIN,

includes the parasitic input common mode capacitance, CCM (3pF), and the input differential

mode capacitance, CDIFF (8pF). CIN acts in parallel with the intrinsic capacitance of the

photodiode, CD. At higher frequencies, the capacitors affect the circuit response. The output

capacitance of a current sensor has a strong effect on the stability of the op-amp feedback

loop.

CF stabilizes the gain and limits the transimpedance bandwidth. To ensure good stability

and to obtain good noise performance, CF can be set as shown in Equation 10.

Equation 10

where,

• CIN = CCM + CDIFF = 11 pF

• CDIFF is the differential input capacitance: 8 pF typical

• CCM is the Common mode input capacitance: 3 pF typical

• CD is the intrinsic capacitance of the photodiode

• CSMR is the parasitic capacitance of the surface mount RF resistor: 0.2 pF typical

• FGBP is the gain bandwidth product: 10 MHz at 16 V

RF fixes the gain as shown in Equation 11.

Equation 11

Figure 42. High speed photodiode

CIN CD

π RF FGBP

⋅⋅

⋅

--------------------------------------------------

CSMR

–

VOUT

RF ID

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