DESIGN INFORMATION (Continued)
The information contained in this section has been developed through characterization by Harris Semiconductor and is for use as
application and design information only. No guarantee is implied.
17
HA5022
the enable/disable function. The large value resistors in
series with the DISABLE pin makes it appear as a current
source to the driver. When the driver pulls this pin low cur-
rent flows out of the pin and into the driver. This current,
which may be as large as 350µA when external circuit and
process variables are at their extremes, is required to insure
that point “A” achieves the proper potential to disable the
output. The driver must have the compliance and capability
of sinking all of this current.
FIGURE 38. SIMPLIFIED SCHEMATIC OF ENABLE/DISABLE
FUNCTION
When VCC is +5V the DISABLE pin may be driven with a
dedicated TTL gate. The maximum low level output voltage
of the TTL gate, 0.4V, has enough compliance to insure that
the amplifier will always be disabled even though D1 will not
turn on, and the TTL gate will sink enough current to keep
point “A” at its proper voltage. When VCC is greater than +5
volts the DISABLE pin should be driven with an open collec-
tor device that has a breakdown rating greater than VCC.
Referring to Figure 8, it can be seen that R6 will act as a
pull-up resistor to +VCC if the DISABLE pin is left open. In
those cases where the enable/disable function is not
required on all circuits some circuits can be permanently
enabled by letting the DISABLE pin float. If a driver is used
to set the enable/disable level, be sure that the driver does
not sink more than 20µA when the DISABLE pin is at a high
level. TTL gates, especially CMOS versions, do not violate
this criteria so it is permissible to control the enable/disable
function with TTL.
Two Channel Video Multiplexer
Referring to the amplifier U1A in Figure 39, R1 terminates
the cable in its characteristic impedance of 75Ω, and R4
back terminates the cable in its characteristic impedance.
The amplifier is set up in a gain configuration of +2 to yield
an overall network gain of +1 when driving a double termi-
nated cable. The value of R3 can be changed if a different
network gain is desired. R5 holds the disable pin at ground
thus inhibiting the amplifier until the switch, S1, is thrown to
position 1. At position 1 the switch pulls the disable pin up to
the plus supply rail thereby enabling the amplifier. Since all
R6
15K
R7
15K
+VCC
ENABLE/DISABLE INPUT
D1
QP3
R8
QP18
A
R33
R10
of the actual signal switching takes place within the amplifier,
it’s differential gain and phase parameters, which are 0.03%
and 0.03 degrees respectively, determine the circuit’s perfor-
mance. The other circuit, U1b, operates in a similar manner.
When the plus supply rail is 5V the disable pin can be driven
by a dedicated TTL gate as discussed earlier. If a multiplexer
IC or its equivalent is used to select channels its logic must
be break before make. When these conditions are satisfied
the HA5022 is often used as a remote video multiplexer, and
the multiplexer may be extended by adding more amplifier
ICs.
Low Impedance Multiplexer
Two common problems surface when you try to multiplex
multiple high speed signals into a low impedance source
such as an A/D converter. The first problem is the low source
impedance which tends to make amplifiers oscillate and
causes gain errors. The second problem is the multiplexer
which supplies no gain, introduces all kinds of distortion and
limits the frequency response. Using op amps which have an
enable/disable function, such as the HA5022, eliminates the
multiplexer problems because the external mux chip is not
needed, and the HA5022 can drive low impedance (large
capacitance) loads if a series isolation resistor is used.
Referring to Figure 40, both inputs are terminated in their
characteristic impedance; 75Ω is typical for video applica-
tions. Since the drivers usually are terminated in their char-
acteristic impedance the input gain is 0.5, thus the
amplifiers, U2, are configured in a gain of +2 to set the circuit
gain equal to one. Resistors R2 and R3 determine the ampli-
fier gain, and if a different gain is desired R2 should be
changed according to the equation G = (1 + R3/R2). R3 sets
the frequency response of the amplifier so you should refer
to the manufacturers data sheet before changing it’s value.
R5, C1 and D1 are an asymmetrical charge/discharge time
circuit which configures U1 as a break before make switch to
prevent both amplifiers from being active simultaneously. If
this design is extended to more channels the drive logic
must be designed to be break before make. R4 is enclosed
in the feedback loop of the amplifier so that the large open
loop amplifier gain of U2 will present the load with a small
closed loop output impedance while keeping the amplifier
stable for all values of load capacitance.
The circuit shown in Figure 40 was tested for the full range
of capacitor values with no oscillations being observed; thus,
problem one has been solved. The frequency and gain char-
acteristics of the circuit are now those of the amplifier inde-
pendent of any multiplexing action; thus, problem two has
been solved. The multiplexer transition time is approximately
15µs with the component values shown.