Texas Instruments ADS8481EVM User Manual page 3

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The noise coming through the driver amplifier is filtered by a single-pole filter using R = 12 Ω and C = 1 nF
with a corner frequency of 13 MHz. The 12-Ω series resistor works with the capacitor to filter the input
signal, but also isolates the amplifier from the capacitive load. The 1000-pF capacitor to ground at the
input of the ADC works with the series resistor to filter the input signal, behaves like a charge reservoir,
and provides a path to ground for high-frequency noise and the input current transient which occurs when
the device switches from hold to sample mode. This larger external filter capacitor works with the amplifier
to charge the internal sampling capacitor during sampling mode.
The supplies to the input amplifier are selectable with solder jumper pad SJP3. When deciding on supply
rails, a good rule is to add at least 2 V of head room on either side to achieve optimal performance for
dual supply amplifiers. For example, if the signal applied to the amplifier is 0 V to 4 V, then the amplifier
rails should be at least –2 V and +6 V. At the 16-bit and 18-bit performance nodes, the
amplifier-introduced distortion can become significant if the amplifier is not given adequate headroom. Be
aware when reading amplifier data sheets that they may not be specified with large amplitudes. Therefore,
it may not be possible to surmise from a cursory glance how well the amplifier will behave in a system.
The default configuration from the factory is to set the input for translating a bipolar ±2-V signal to 0 V to 4
V. The necessary DC component to offset the signal is generated by U4. U4 is the low-noise THS4032
amplifier. If necessary, it can be configured to further filter the reference voltage IC, U2, and provide
positive or negative gains. The ADS8481EVM leaves the factory with potentiometer, R24, set to 1.024 V.
Table 1 indicates how the solder pad jumpers should be set to select for the various supply and input
options for the analog driver circuitry. The schematic for the ADS8481EVM appears in Appendix C.
300 W
V
I
± 2 Vpp
R35
1.024 V
SLAU182A – June 2006 – Revised September 2006
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300 W
R36
-12 V
1 F
m
C12
4
3
6
THS4031
2
1 F m
7
U6
C11
+12 V
Figure 1. Input Buffer Circuit
12
12 W
R39
1 nF
C45
ADS8481EVM
Analog Interface
(+) IN
0-4 V
(-) IN
3