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ADIAD7685 High Performance Process Control Analog Front End Solution

ADI’s AD7685 is a 16-bit charge redistribution successive approximation analog-to-digital converter (ADC), single-supply 2.3V-5.5V operation, including a low-power high-speed 16-bit sampling ADC with no missing codes, an internal conversion clock, and multiple Function serial port. Throughput 250kSPS, INL typical ±0.6 LSB, maximum ±2 LSB ((±0.003% of FSR)), 20kHz SINAD is 93.5dB, THD is -110dB, false differential analog input, mainly used in Handheld devices, medical instruments, mobile communications, PDAs, data acquisition and instrumentation in process control. This paper introduces the main features of AD7685, power supply block diagram for multiple power applications, true 16-bit isolated simultaneous sampling acquisition system block diagram, EVAL-AD76XXCB 8/10 -Pin evaluation board main features, circuit diagram and simplified block diagram of a complete high speed high CMRR precision analog front end for process control, EVAL-A-INPUT-1AZ evaluation board circuit diagram and bill of materials.

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 main features:

16-bit resolution with no missing codes

Throughput: 250 kSPS

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

SINAD: 93.5dB @ 20kHz

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

1.8V to 5V 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 application:

Battery-powered equipment

Medical instruments

mobile communications

Personal digital assistants (PDAs)

Data acquisition

Instrumentation

Process controls
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 1. ADC simplified circuit diagram
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 2. AD7685 Multiple Power Supply Application Power Block Diagram
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 3. Block Diagram of AD7685 True 16-Bit Isolated Simultaneous Sampling Acquisition System

EVAL-AD76XXCB 8/10-Pin Evaluation Board

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 Evaluation Board Key Features:

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
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 4. EVAL-AD76XXCB 8/10-Pin Evaluation Board Circuit Diagram: Analog Section
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 5. EVAL-AD76XXCB 8/10-Pin Evaluation Board Circuit Diagram: Digital Section
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 6. EVAL-AD76XXCB 8/10-Pin Evaluation Board Circuit Diagram: Power Section
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 7. EVAL-AD76XXCB 8/10-Pin Evaluation Board Circuit Diagram: Selection Section

High speed and high CMRR precision analog front end for complete process control

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.
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 8. Simplified block diagram of an analog front end for high performance process control
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 9. EVAL-A-INPUT-1AZ Evaluation Board and SDP Board Connection Diagram
ADIAD7685 High Performance Process Control Analog Front End Solution
Figure 10. EVAL-A-INPUT-1AZ Evaluation Board Circuit Diagram (1)

Figure 11. EVAL-A-INPUT-1AZ Evaluation Board Circuit Diagram (2)
For the EVAL-A-INPUT-1AZ evaluation board bill of materials see:

For details, see:
http://www.analog.com/static/imported-files/data_sheets/AD7685.pdf
and
http://www.analog.com/static/imported-files/circuit_notes/CN0213.pdf

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