Texas Instruments ADS848 EVM Series User Manual page 3

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3.1 Input Signal Conditioning
The analog input circuitry, consisting of three
passive components to configure the EVM for positive or negative gains, as well as input range scaling,
filtering, and level translation (for example, adding a dc offset). The installed operational amplifiers are
housed in an industry-standard SOIC footprint. This architecture enables the user to replace the THS4031
with a wide range of dual- and single-supply amplifiers housed in an SOIC package. When choosing the
driver amplifier, the user should consider the following requirements: The driving amplifier must be able to
settle the input to 18-bit level (0.00038%) within the sample time of the converter. It should have total
harmonic distortion (THD) characteristics comparable to or better than the ADS848x over the bandwidth of
interest. Lastly, the noise generated by the amplifier must be as low as possible, so as not to degrade (or
source-limit) the performance of the ADS848x.
The RC circuit, at the input of the ADC, filters the input signal and helps charge the ADC sample-and-hold
stage. The noise coming through the driver amplifier is also filtered using this single-pole filter. The 5-Ω
series resistor works with the capacitor to filter the input signal, but also isolates the amplifier from the
capacitive load. The 2200-pF capacitor works with the series resistor to filter the input signal and behaves
like a charge reservoir, providing a discharge path for high-frequency noise and the input current
transients that occur when the device switches from hold to sample mode. This external capacitor works
with the amplifier to charge the internal sampling capacitor during sampling mode. In multiplexing
applications, the values of these resistors and capacitor may need to be adjusted. As with the driving
amplifier, the RC circuit must also be able to settle the signal to a 18-bit level within the sample time.
The supplies to the input amplifier are selectable with solder jumper pad SJP3 and SJP4. Shorting across
pads 1 and 2 will ground the negative rail. Shorting across pads 2 and 3 will tie the negative supply of the
amplifiers to the voltage applied at node –VCC.
When deciding on supply rails for bipolar amplifiers, a good rule is to add at least 2 V of head room on
either side to achieve optimal performance. For example, if the signal applied to the amplifier is 0V to 4 V,
then the amplifier rails should be at least –2 V and +6 V. For the 16-bit and 18-bit performance nodes, the
amplifier-introduced distortion can become significant and degrade overall system perforamnce. For
CMOS, or single-supply amplifiers, this configuration is not always possible. As with the bipolar amplifiers,
single-supply amplifiers also degrade with larger amplitudes and at higher input frequencies. The user
must test the amplifier separately to understand its respective characteristics across the given input
conditions. Take care when reading product data sheets to understand how the device is characterized. It
may not be possible to surmise from a cursory glance how well the amplifier will behave in a given
system.
Single-ended signal sources are readily available; therefore, the ADS848xEVM ships in this configuration.
The factory-set default configuration of the input driving circuitry is for converting a single-ended, unipolar
signal to differential. The necessary dc component to offset the signal at U7 is generated by U4. U4 is the
low-noise THS4032
provide positive or negative gains. The ADS848xEVM leaves the factory with potentiometer, R24, set to
2.048 V and set as shown in
the input circuitry be set up as shown in
Table 2 indicates how the solder pad jumpers should be set to select from the various supply and input
options for the analog driver circuitry. The schematic for the ADS848xEVM is attached at the end of this
document.
SLAU186A – August 2006 – Revised September 2008
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amplifier. It can be configured to further filter the reference voltage IC, U2, and
Figure 1. If a fully differential signal source is available, it is recommended
Figure 2
THS4031 operational amplifiers, allow the user to install
or as Figure 40 in the
Analog Interface
ADS8484 product data sheet.
ADS848xEVM 3