Description
1-2
Introduction and Description
1.1
Description
This EVM is made of the THS4121 high-speed operational amplifier, a number
of passive components, and various features and component footprints that
enable the user to experiment and verify various operational amplifier circuit
configurations.
Initially, this board is populated for a single-ended
input/differential-output amplifier (see Figure 1-2 for populated circuits). The
outputs (V
O+
and V
O–
) can be tested differentially or single ended. Gain is set
to one and can be changed by changing the ratios of the feedback and gain
resistors (see the device datasheet for recommended resistor values). Each
input is terminated with a 50-
Ω
resistor to provide correct line-impedance
matching.
This EVM is optimized to work with
±
1.65 power supplies. If the user tests the
device in a single supply mode (V
DD
= 3.3 V), then the proper grounding should
be applied. The V
OCM
voltage level would be set at the mid rail (1.65-V when
V
DD
= 3.3 V). In general, the mid rail is the optimal area of the operation for
amplifiers. Therefore, if the device is operating at V
DD
= 3.3 V, it is
recommended that a dc offset be added to the input signal to shift it to the mid
rail. Having the input signal centered at the mid rail reduces the peaking effect.
RPU resistors in this EVM work as input signal level shifters. Therefore, they
can be used to shift the input signal level. To calculate the RPU values use the
following equation.
V
(p)
= V
(pullup)
, RPU = R pullup (see EVM schematic)
RPU
+
V
(
p
)
*
V
DD
Ă(
V
I
*
V
(
P
)
)
1
Rg
)
(
V
O
*
V
(
p
)
)
1
Rf
The above equation is a good estimation tool for RPU. The user should
consider the component error contribution to the system. As a result, the RPU
number generated by the above equation could be changed slightly to
compensate for the error to provide the optimal result.
Having V
OCM
level at 1.65 V above the GND causes a current flow from V
OCM
to GND. Consideration should be given to this current flow which will increase
the I
DD
.
1.2
Evaluation Module Features
THS4121 high-speed operational amplifier EVM features include:
J
Single and differential input and output capability
J
Nominal 50-
Ω
input and output termination resistors. They can be
configured according to the application requirement.
J
V
OCM
direct input control (see schematic and the device data sheet)
J
V
OCM
pin can be controlled via transformer center-tap (see
schematic)
J
Shutdown pin control, JU1 (if applicable to the device, see the device
data sheet)
J
Input and output transformer footprints for changing single-ended
signals to differential signals
