TSX921 datasheet(22/32 Pages) STMICROELECTRONICS

22 / 32 page

Application information

TSX920, TSX921, TSX922, TSX923

22/32

DocID024310 Rev 4

5.8

High-speed photodiode

The TSX92x 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 the TSX92x operational amplifiers 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

VOUT = RF x ID

Figure 42: High-speed photodiode

TSX921 データシート(PDF) 22 Page - STMicroelectronics