原标题：ADI AD7685高性能过程控制模拟前端解决方案

　　ADI公司的AD7685是16位电荷重新分布逐次逼近模数转换器(ADC),单电源2.3V-5.5V工作,包括有不丢失码的低功耗高速16位取样 ADC,内部转换时钟,以及多功能串口.吞吐量250kSPS, INL典型为±0.6 LSB,最大为±2 LSB((±0.003% of FSR)),20kHz的SINAD为93.5dB,THD为-110dB,假性差分模拟输入,主要用在手持设备,医疗仪器,移动通信,PDA,数据采集和仪器仪表于过程控制.本文介绍了AD7685主要特性, 多个电源应用电源框图, 真正16位隔离同时取样采集系统框图,EVAL-AD76XXCB 8/10-Pin评估板主要特性,电路图以及完整的过程控制用高速高CMRR精密模拟前端简化框图, EVAL-A-INPUT-1AZ评估板电路图和材料清单.

　　The AD7685 is a 16-bit, charge redistribution successive approximation, analog-to-digital converter (ADC) that operates from a single power supply, VDD, between 2.3 V to 5.5 V. It contains a low power, high speed, 16-bit sampling ADC with no missing codes, an internal conversion clock, and a versatile serial interface port. The part also contains a low noise, wide bandwidth, short aperture delay, track-and-hold circuit. On the CNV rising edge, it samples an analog input IN+ between 0 V to REF with respect to a ground sense IN−. The reference voltage, REF, is applied externally and can be set up to the supply voltage. Power dissipation scales linearly with throughput.

　　The SPI-compatible serial interface also features the ability, using the SDI input, to daisy chain several ADCs on a single 3-wire bus or provides an optional BUSY indicator. It is compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using the separate supply VIO.

　　The AD7685 is housed in a 10-lead MSOP or a 10-lead QFN (LFCSP) with operation specified from −40°C to +85°C.

　　AD7685主要特性:

　　16-bit resolution with no missing codes

　　Throughput: 250 kSPS

　　INL: ±0.6 LSB typical, ±2 LSB maximum (±0.003% of FSR)

　　SINAD: 93.5 dB @ 20 kHz

　　THD: −110 dB @ 20 kHz

　　Pseudo differential analog input range

　　0 V to VREF with VREF up to VDD

　　No pipeline delay

　　Single-supply operation 2.3 V to 5.5 V with

　　1.8 V to 5 V logic interface

　　Serial interface SPI®-/QSPI™-/MICROWIRE™-/DSP-compatible

　　Daisy-chain multiple ADCs, BUSY indicator

　　Power dissipation

　　1.4 μW @ 2.5 V/100 SPS

　　1.35 mW @ 2.5 V/100 kSPS, 4 mW @ 5 V/100 kSPS

　　Standby current: 1 nA

　　10-lead package: MSOP (MSOP-8 size) and

　　3 mm × 3 mm QFN (LFCSP) (SOT-23 size)

　　Pin-for-pin-compatible with 10-lead MSOP/QFN PulSAR® ADCs

　　AD7685应用:

　　Battery-powered equipment

　　Medical instruments

　　Mobile communications

　　Personal digital assistants (PDAs)

　　Data acquisition

　　Instrumentation

　　Process controls

　　图1.ADC简化电路图

　　图2.AD7685 多个电源应用电源框图

　　图3.AD7685真正16位隔离同时取样采集系统框图

　　EVAL-AD76XXCB 8/10-Pin评估板

　　The EVAL-AD76XXCB 8/10-Pin is an evaluation board for the AD768x/AD769x/AD794x/AD798x 8 and 10-pin PulSAR high resolution ADCs (see the Ordering Guide at the end of this document for a product list).

　　The evaluation board is designed to demonstrate the ADC’s performance and to provide an easy to understand interface for a variety of system applications.

　　The evaluation board is ideal for use with either Analog Devices EVAL-CONTROL BRD2/BRD3 (EVAL-CONTROL BRDx), DSP based controller board, to run the Analog devices evaluation software and to develop a specific application using LabVIEW, or as a stand-alone evaluation board.

　　The EVAL-CONTROL BRDx is sold separately from the evaluation board, is required to run the evaluation software, is not required in stand alone mode and can be reused with many Analog Devices ADCs.

　　EVAL-AD76XXCB 8/10-Pin评估板主要特性:

　　Versatile analog signal conditioning circuitry

　　On-board reference, crystal oscillator and buffers

　　16-bit Parallel Buffered Outputs

　　Ideal for DSP and data acquisition card interfaces

　　Analog and digital prototyping area for breadbording the

　　target system

　　Stand-alone operation or Eval control board compatibility

　　PC software for control and data analysis

　　LabVIEW1 driver to develop custom application

　　图4. EVAL-AD76XXCB 8/10-Pin评估板电路图:模拟部分

　　图5. EVAL-AD76XXCB 8/10-Pin评估板电路图:数字部分

　　图6. EVAL-AD76XXCB 8/10-Pin评估板电路图:电源部分

　　图7. EVAL-AD76XXCB 8/10-Pin评估板电路图:选择部分

　　完整的过程控制用高速高CMRR精密模拟前端

　　Complete High Speed, High CMRR Precision Analog Front End for Process Control

　　Signal levels in industrial process control systems generally fall into one of the following categories: single-ended current (4 mA-to-20 mA), single-ended, differential voltage (0 V to 5V, 0 V to 10 V, ±5 V, ±10 V), or small signal inputs from sensors such as thermocouples or load cells. Large common-mode voltage swings are also typical, especially for small signal differential inputs; therefore good common-mode rejection is an important specification in the analog signal processing system.

　　The analog front-end circuit shown in Figure 1 is optimized for high precision and high common-mode rejection ratio (CMRR) when processing these types of industrial-level signals.

　　图8.高性能过程控制的模拟前端简化框图

　　图9. EVAL-A-INPUT-1AZ评估板和SDP板连接图