5. USING THE L-17C
5.1
GENERAL GUIDANCE
The L-17C is remarkably tolerant of layout. It will, in fact, work in ZIF sockets without serious
degradation of performance. This is very useful for testing ICs. However, for optimum
performance, a compact layout with a solid ground-plane is required.
There is one area where care is needed, and that is the geometric configuration of the V
CC
lines.
These must be star-connected, i.e. use separate lines from the regulator to each V
CC
input on the
IC, and, if possible, a third line to external V
CC
connections, e.g. the voltage desensitizer. For
compact layouts, a 4.7
F cap at the regulator, a 0.1
F right at pin 23 (V
CC
2), and a 1
F at pin 7
(V
CC
1) should suffice. For larger layouts, additional decoupling at remote V
CC
connections may be
necessary.
The traces from both voltage regulators to the IC should be wide, and small decoupling caps (1 to
10 nF) should also be connected close to each V
EE
power input on the IC. Finally, for DLVA
applications, the detector should be placed as close as possible to the input, or a coaxial
connection or strip line must be used to make the connection from the detector to the input.
The ANADYNE L-17C PC36-1 test and evaluation board is, for convenient use, much larger than
any actual board needs to be for any single application, since it has provisions to test all features
of the IC. Also, this board does not require any surface-mount components or 1/10-watt resistors,
since it has been designed for use with readily- available components.
We have found it useful to install socket-pins for resistors that may require changing during the
R&D phase on a new layout. This enables the resistors to be changed without damaging the
board. When a satisfactory configuration is achieved, the resistors can then be hard soldered.
We have also found it helpful to have capacitors in useful ranges glued to popsicle sticks (with the
leads projecting from one end) for quick tests when pulse-shaping and for tracking down any
oscillations. This is a quick and convenient way to probe, and prevents excess capacitance
(fingers) from affecting the results.
NOTE: ALL CAPACITORS EXCEPT DECOUPLING CAPS MUST BE
TEMPERATURE STABLE; DO NOT USE CERAMIC CAPS (EXCEPT
FOR TUBULAR CERAMICS WITH ZERO TEMPCO); DO NOT USE
POTS WHEN TEMPERATURE TESTING - THEY EXHIBIT INSTABILITY.
VOLTAGE REGULATION
Good voltage regulators are essential. In order to achieve
1/2 dB stability over temperature, the
regulators should track to within 15 mV over the same temperature range. We recommend LM723
or LM117 - LM137 voltage regulators. Since the gain of the IC is so high when the full dynamic
range is used, the L-17C is sensitive to both AC and DC voltage variations. Many DLVAs require
excessively large decoupling capacitors, typically about 100
F. This can be a serious
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