DEMO MANUAL DC1416 LT1222 and LT1793 Low Noise Transimpedance Amplifier Description Demonstration circuit 1416 is a low noise transimpedance These components are arranged with jumpers allowing amplifier. It utilizes the low voltage noise LT 1222 op amp, various composite configurations. A socketed photodiode, and the low current noise LT1793 op amp, along with OSRAM SFH213, is also provided. the discrete NXP JFET BF862, allowing the user to take Design files for this circuit board are available at advantage of each components particular optimization. DEMO MANUAL DC1416 o perating p Composite amplifiers using single JFET inputs can be accuracy, overriding the high 400mV or so V of the GS classified into two groups: common drain (or source JFET. The standard jumper configuration is Integrator follower) and common source (or JFET in gain). The In, so the TIA will be DC accurate to within the V of the OS standard jumper configuration of this board, as shipped LT1793 (900V max). With the integrator removed from and as indicated on the silkscreen, has the JFET as a the circuit, the DC error rises to 400mV or so. source follower. The feedback resistor, which sets the TIA gain, is 1M. So Another classification is DC accuracy. The LT1793 has the output will respond at 1V per microamp of photocurrent. been provided on board as an integrator to provide DC Quick s tart p D Demonstration circuit 1416 is shipped with the jumpers 4. You should now be able to wave your hand over the set for source follower operation with the integrator in. clear photodiode provided on the board (upper left of If the jumpers have been changed, restore them to the Figure 1) and see the effect in both DC signal and noise. positions shown in the schematic. Refer to Figure 1 for With the photodiode in darkness, or removed from its proper measurement equipment setup and follow the socket, the output should sit near ground. procedure below: 5. You can now connect an optical source to excite the 1. With power off, connect the +12V, 12V, and Com photodiode in a more controlled manner. The easiest + leads from the power supply to the V , V , and GND way is to drive an LED directly from a function genera- terminals of the demo circuit, as shown in Figure 1. tor, with the function generators internal 50 source impedance as a current limiter. Any standard color or 2. With power off, connect the V of the demo circuit OUT IR LED can be used, as the photodiode provided has to an oscilloscope or DMM. You can use either the a wide sensitivity. Be careful not to overdrive sensitive gold SMA connector or the turrets provided on board, devices such as small lasers. or both. Set a high range such as 2V/DIV on the oscil- loscope, or V on the DMM. 6. You can now decide whether to play with other configura- DC tions (JFET in gain, integrator out, more compensation, 3. Turn on the power supply. etc), or to replace the provided photodiode with the one you intend to use. dc1416f 2 ure roce rinciples