Description
1-5
General Information
The THS7001 EVM is equipped with BNC input and output connectors. The
input is terminated with a 50-
Ω
resistor to provide correct line impedance
matching (Figure 1–3). Note that using a source with a 50-
Ω
output impedance
will create a voltage divider at the EVM inputs. Thus, accurate knowledge of
the source output characteristics is required to determine proper input signal
amplitudes.
The output of the preamplifier stage is routed through a 50-
Ω
resistor to
provide proper cable impedance matching and termination impedance
matching. One of the drawbacks of using this type of impedance matching is
that it places an equivalent 100-
Ω
load on the output of the preamplifier.
Although the preamplifier section has a large output current capability, the
programmable gain amplifier (PGA) section has a limited current drive
capability. It is recommended that the total impedance placed on the output of
the PGA section is at least 500
Ω
.
The THS7001 IC is a voltage feedback amplifier. It follows the classic
operational amplifier gain equations:
Inverting Gain
+
*
R
F
R
G
Noninverting Gain
+
1
)
R
F
R
G
The gain of the preamplifier can easily be changed to support different
applications by changing the resistor ratios. Although any of the components
on the EVM board can be replaced with different values, it is imperative that
the THS7001 preamplifier gain be kept to a minimum of +2 or –1 for stability
purposes. Also, component pads have been placed in convenient locations on
the PCB (shown as components with the value
TBD in the schematic) to allow
numerous modifications to the basic EVM configuration. However, care must
be taken because the surface-mount solder pads on the board are somewhat
fragile and will not survive a large number of soldering/desoldering operations.
For independent evaluation of the preamplifier, a BNC connector directly at the
preamplifier output pin (J5) allows the user to investigate the preamplifier
independently of the PGA section.
The shutdown feature of the THS7001 IC is implemented on this EVM. The
shutdown signal is low for normal THS7001 operation. When the shutdown pin
is high (5 V), the preamplifier and the PGA section are turned off. Shutdown
on the EVM is controlled by switch S1:D, or by applying an external shutdown
signal to test point TP7 with the switch set to OFF. An isolation resistor on the
THS7001 IC shutdown control input pin is used to minimize surges in the EVM
environment and can be omitted in a final system design.
(1)
(2)
