Datasheet

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1 EVM Overview
1.1 Features
2 Analog Interface
2.1 Signal Conditioning
EVM Overview
• Full-featured evaluation board for the ADS8380 18-bit, 600-kHz, single-channel, high-speed
serial-interface analog-to-digital converter (ADC)
• Onboard signal conditioning
• Onboard reference
• Input and output digital buffer
The ADS8380 analog-to-digital converter has both a positive and negative analog input pin. A signal for
the positive input pin can be applied at connector P1, pin 2 (shown in Table 1 ) or applied to the center pin
of SMA connector J1. The EVM ships with the negative input pin shorted to ground close to the converter
using via SJP7. The negative and positive input pins have a range of -200 mV to +200 mV and -200 mV
to VREF+200 mV, respectively.
Table 1. Analog Input Connector
Description Signal Name Connector Pin No. Signal Name Description
Inverting Input –IN P1.1 P1.2 +IN Noninverting Input
Reserved N/A P1.3 P1.4 N/A Reserved
Reserved N/A P1.5 P1.6 N/A Reserved
Reserved N/A P1.7 P1.8 N/A Reserved
Ground AGND P1.9 P1.10 N/A Reserved
Ground AGND P1.11 P1.12 N/A Reserved
Reserved N/A P1.13 P1.14 N/A Reserved
Ground AGND P1.15 P1.16 N/A Reserved
Ground AGND P1.17 P1.18 N/A Reserved
Reserved N/A P1.19 P1.20 REF+ External Reference Input
It is a recommended practice to buffer the analog input to any SAR-type converter with a high-speed,
fast-settling, low-noise amplifier. The amplifier circuit shown in Figure 1 is the buffer circuit used on the
ADS8380EVM. This circuit consists of the THS4031, a high-speed, low-noise, low-distortion amplifier
configured as an inverting gain of one. The circuit shown in Figure 1 was optimized to achieve the ac
specifications (i.e., SNR, THD, SFDR, etc.) listed in the ADS8380 data sheet. The 12- Ω series resistor
works with the capacitor to filter the input signal, but also isolates the amplifier from the capacitive load.
The 6800-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 short to ground for high-frequency noise and
kickback currents when the device switches from hold to sample mode. This external filter capacitor also
works with the amplifier to charge the internal sampling capacitor during sampling mode.
The type of input capacitors used in the signal path can make a few decibels of difference in ac
performance. Polypropylene or C0G-type capacitors are recommended for the input signal path.
Polypropylene capacitors cause the least distortion of the input signal and have excellent long-term
stability, but are expensive and bulky. C0G ceramic capacitors cost less, come in smaller packages, and
perform as well as polypropylene capacitors in many applications, but are not as stable over time and
temperature. The 68-pF and 6800-pF capacitors installed on the EVM are low-cost C0G type,
manufactured by TDK Corporation.
ADS8380EVM 2 SLAU146 – December 2004