Datasheet

www.ti.com
2 Introduction
3 Analog Interface
3.1 Analog Input – Channel A1
Introduction
The ADS7864 contains two high-speed 12-bit A/D converters that operate from independent +5-V Avdd
and Dvdd supplies. Each A/D in the chip is preceded by a 3:1 multiplexer, providing six total input
channels. The parallel digital output is provided as a 16-bit word which contains both channel address
information and conversion data results.
Each of the six input channels contains a fully differential sample-and-hold circuit arranged into three pairs
(A, B, and C). Each channel pair has a hold signal (HOLDA, HOLDB, and HOLDC) which, when strobed
together, allows simultaneous sampling of all six analog inputs. The device accepts an analog input
voltage in the range of –V
REF
to +V
REF
. The ADS7864 also accepts bipolar input ranges when a level shift
circuit is used in the analog front-end circuitry (see Figure 1 ).
Conversion time for the ADS7864 is 1.75 µ s when an 8-MHz external clock is used. The corresponding
acquisition time is 0.25 µ s. To achieve maximum output rate (500 kHz), the read function can be
performed at the start of the next conversion cycle.
The analog input to the ADS7864 is divided in two parts. Connector J4 provides access to input channels
A1 and B1 through two different amplifier circuit configurations. The input buffer configuration of channel
A1 presents a typical front-end circuit for the A/D converter. Its function is to provide level and impedance
adaptation of the input signal. The input to channel B1 is a bipolar configuration using the INA159 to
accommodate ± 10-V input signals. Connector J3 provides access to the remaining analog input channels
through a simple R/C filter.
The analog input to the ADS7864MEVM board for channel A1 is comprised of the dual OPA2132
operational amplifier and the associated circuitry as shown in Figure 1 . The OPA2132 is powered from the
± 12-V analog supply, and arranged as an inverting amplifier with a gain of 1. The internal +2.5-V
reference voltage of the ADS7864 is applied to the noninverting input of the OPA2132 to provide input
bias.
NOTE: Components marked NI are NOT INSTALLED.
Figure 1. ADS7864MEVM Schematic – CHA1 Input
2 ADS7864MEVM SLAU188 – September 2006
Submit Documentation Feedback