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dc1416fa
DEMO MANUAL DC1416
OPERATING PRINCIPLES
Composite amplifiers using single JFET inputs can be
classified into two groups: common drain (or “source
follower”) and common source (or “JFET in gain”). The
standard jumper configuration of this board, as shipped
and as indicated on the silkscreen, has the JFET as a
source follower.
Another classification is DC accuracy. The LT1793 has
been provided on board as an integrator to provide DC
accuracy, overriding the high 400mV or so V
GS
of the
JFET. The standard jumper configuration is “Integrator
In”, so the TIA will be DC accurate to within the V
OS
of the
LT1793 (900µV max). With the integrator removed from
the circuit, the DC error rises to 400mV or so.
The feedback resistor, which sets the TIA gain, is 1MΩ. So
the output will respond at 1V per microamp of photocurrent.
QUICK START PROCEDURE
Demonstration circuit 1416 is shipped with the jumpers
set for source follower operation with the integrator in.
If the jumpers have been changed, restore them to the
positions shown in the schematic. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
1. With power off, connect the +12V, –12V, and Com
leads from the power supply to the V
+
, V
–
, and GND
terminals of the demo circuit, as shown in Figure 1.
2. With power off, connect the V
OUT
of the demo circuit
to an oscilloscope or DMM. You can use either the
gold SMA connector or the turrets provided on board,
or both. Set a high range such as 2V/DIV on the oscil-
loscope, or V
DC
on the DMM.
3. Turn on the power supply.
4. You should now be able to wave your hand over the
clear photodiode provided on the board (upper left of
Figure 1) and see the effect in both DC signal and noise.
With the photodiode in darkness, or removed from its
socket, the output should sit near ground.
5. You can now connect an optical source to excite the
photodiode in a more controlled manner. The easiest
way is to drive an LED directly from a function genera-
tor, with the function generator’s internal 50Ω source
impedance as a current limiter. Any standard color or
IR LED can be used, as the photodiode provided has
a wide sensitivity. Be careful not to overdrive sensitive
devices such as small lasers.
6. You can now decide whether to play with other configura-
tions (JFET in gain, integrator out, more compensation,
etc), or to replace the provided photodiode with the one
you intend to use.
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