Page 59 - Analog Circuit Design Art, Science, and Personalities

Reflections of a Dinosaur

cation of the transistor-transistor switch (Figure 612) to the transistor-diode switch
(Figure 6-3) is an illustration of replacing a transistor with a diode to reduce cost
and save space. If switching time is not a consideration, one would choose a binary
weighted DAC (Figures 6-2 and 6-3) over the current divider configuration (Figures
6-4 and 6-5) since fewer resistors are required. The value of the resistors is not
important when working with discrete components since all resistors of the same
power rating are the same physical size. A 1/4 W 10 R resistor is the same size as a
1/4 W 100 MR resistor. Since the least number of components minimizes size, the
circuit with the least number of resistors is preferred. The “minimum component
count” strategy is the one to use when the assembly is discrete components on a
printed circuit board, but when chip and wire hybrid construction is used, a different
approach is necessary.
Chip and wire hybrid assemblies are constructed by attaching individual semi-
conductor dice to a ceramic substrate. The surface of the ceramic substrate contains
a gold conductor pattern that interconnects the semiconductor dice attached to the
substrate. Electrical connections are made from the pads of the semiconductor dice
to the gold conductors on the substrate with gold wire (see Figure 6-10).
Precision resistors for the hybrid are made by depositing a thin film of resistive
material such as nickel-chromium on a silicon wafer. Using standard semiconductor
technology, the unwanted resistive film is etched away, leaving the desired resistor
geometries on the silicon wafer. The wafer, with many identical circuits on it, is cut
into individual dice. Each resistor die contains from one to several dozen resistors,
depending on the design. The resistance value of the thin film resistor is determined
by its geometry. The larger the value of the resistor the more area it occupies on the
silicon die. Therefore, the area occupied by resistors in a chip and wire hybrid is
determined by the total resistance and not by the number of resistors. In a chip
and wire hybrid, the total resistance should be minimized to keep the unit as small
as possible.
The size advantage gained with discrete components by using a diode instead of
a transistor is lost in a chip and wire hybrid assembly. A transistor die is approxi-
mately the same size and cost as a diode die. In fact, when the circuit requires a
diode, it can be obtained by using a transistor connected as a diode. The base and
collector of the transistor are connected together to form one terminal of the diode,
and the emitter of the transistor is the other terminal of the diode. Using a transistor
to replace the diode can also help your purchasing department by reducing the
number of different components it has to buy.
It can be concluded from the last two paragraphs that the circuit topology used
for printed circuit construction is not optimum for a chip and wire hybrid. Printed
circuit construction places a high priority on minimizing the number of resistors to
reduce the size of the unit. The total value of the resistance is the parameter that
determines the size of a thin film resistor die used in a hybrid. For example, five
10 KR resistors on a thin film die occupy less than l/lOth the area that one 500 KR
thin film resistor die will occupy. But five 10 KR discrete resistors on a printed
circuit board will occupy five times the space of one 500 KR discrete resistor. To
minimize the size of a chip and wire hybrid, one must minimize the total resistance,
even though a greater number of resistors is used.
The optimum topology for a 12-bit chip and wire hybrid DAC is different from
the optimum topology for a discrete component version of a 12-bit DAC. Table 6-1
shows the number of resistors required in the switching section for both the binary
weighted and the current division DACs constructed using discrete components on
a printed circuit board and the total resistance for both versions constructed using
chip and wire hybrid technology.

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