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Low Output Impedance MUX (HA5022)
Two common problems will surface when trying to multiplex
multiple high-speed signals into a low-impedance load, such
as an analog-to-digital converter. The first is the low load
impedance, which tends to make amplifiers oscillate and
thus causes gain errors. The second problem involves the
multiplexer, which supplies no gain, introduces distortion,
and limits the frequency response.
Using op amps that have an enable/disable function, such as
the HA5022, will eliminate the multiplexer problems. That’s
because the external multiplexer chip isn’t needed, and the
HA5022 can drive low-impedance (large capacitance) loads
if a series isolation resistor is employed.
Looking more closely at the circuit, both inputs are termi-
nated in their characteristic impedance; 75Ωis typical for
video applications, see Figure 1. Because the output cables
usually are terminated in their characteristic impedance, the
gain is 0.5. Consequently, amplifiers U2A and U2B are con-
figured in a gain of +2 to set the circuit gain at 1. R2and R3
determine the amplifier gain; if a different gain is desired, R2
should be changed according to the equation:
G = (1 + R3/R2).
R3sets the amplifier’s frequency response, so it’s best to
check the manufacturer’s data sheet before changing its
value.
R5,C
1, and D1make up an asymmetrical charge/discharge
time circuit that configures U1 as a break-before-make
switch to prevent both amplifiers from being active simulta-
neously. If this design is extended to more channels, the
drive logic must be designed to be break-before-make. Also,
the inhibit input is only functional when the channel switch
input is high. R4is 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 was tested for the full range of capacitor
values with no oscillations observed. Thus, the problem is
solved. The circuit’s frequency and gain characteristics are
now those of the amplifier independent of any multiplexing
action. This essentially solves the second problem. The mul-
tiplexer transition time is approximately 15µs with the com-
ponent values shown.
FIGURE 1. THIS LOW-OUTPUT IMPEDANCE MULTIPLEXER WILL SOLVE PROBLEMS OF OSCILLATION CAUSED BY LOW LOAD
IMPEDANCE, AS WELL AS DISTORTION AND LIMITED FREQUENCY RESPONSE INTRODUCED BY THE MULTIPLEXER. THE
SECOND PROBLEM IS SOLVED BECAUSE THE FREQUENCY AND GAIN CHARACTERISTICS BECOME THOSE OF THE
AMPLIFIER, INDEPENDENT OF THE MULTIPLEXER.
INPUT B
+
-
-5V
+
-
+5V
INHIBIT
CHANNEL
SWITCH
INPUT A
R1A
75
R1B
75
D1A
1N4148
U1C
U1A U1B U1D
R6
100K
R5A
2000
C1A
0.047µF
R5B
2000
D1B
1N4148
R2A
681 1
2
34
16
R3A
681
R4A
27
0.01µF
R2B
681 R4B
27
R3B
681
0.01µF
OUTPUT
7
6513
10
U2B
U2A
C1B
0.047µF
NOTES:
1. U2: HA5022
2. U1: CD4011
Application Note November 1996 AN9503.1
2
All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification.
Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time with-
out notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
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Application Note 9503
