EVM Analog Input
2-2
2.1
EVM Analog Input
The EVM analog input signal is applied to BNC connector J4 by one of four
methods:
-
Direct
-
Amplifier input (dc-coupled)
-
Amplifier input (ac-coupled)
-
User supplied input
2.1.1
Direct Input
To route the signal directly to the TLC5540/TLC5510/TLC5510A input, solder
jumpers from E7 to E8 and from E23 to E12. This provides a 50-
Ω
load (R5)
at input connector J4. The input signal must be dc-biased to the specifications
defined by the data sheet. The TLC5540/TLC5510/TLC5510A is connected
using its internal bias resistors. Jumper J6 bypasses one of the internal bias
resistors and thus alters the input bias range required by the input signal. With
J6 installed, the voltage range of the TLC5540/TLC5510 is 0 V to 2.28 V (at
3.3 V
DDA
).
The TLC5510A uses only the center internal bias resistor with an externally-
applied regulated 4-V reference to generate the device reference voltage.
Hence the input signal range applied to J4 can be between 0 V–4 V.
The input signal is band-limited to 12 MHz by the LC filter consisting of FB5,
R15, and C5. For a bandwidth of 20 MHz, select a value for C5 in the 15 pF
to 33 pF range.
2.1.2
Amplifier Input, DC-Coupled
A THS3001 high-speed transconductance operational amplifier provides
buffering for dc-coupled amplifier-input signals. The amplifier circuit provides
a flat response to 300 MHz with a gain of two. It can drive a low impedance
load.
The values of R6 and R8 set the amplifier gain to two.
The gain can be reduced to one by removing R6. Since the inverting input is
a low-impedance current-controlled input, R8 must remain in the circuit, and
its value must be changed to 1 k
Ω
. This resistance value is critical because it
controls the high frequency response of the circuit. The TLC5510A amplifier
gain is set to one.
The output rolloff filter, consisting of R4 and C14, provides a small amount of
filtering against frequencies in excess of 20 MHz (f
s
/2). The value of C14 can
be altered to change the filter characteristics, or C14 can be removed entirely.
In most cases, R4 should be retained to lower the direct capacitive load on the
operational amplifier, thereby avoiding high frequency peaking of the output
signal.
Resistor R7 also provides isolation against a direct capacitive load (such as
a scope probe) on the test point terminal.
