AD7616BSTZ datasheet(24/51 Pages) AD

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Data Sheet

AD7616

Rev. 0 | Page 23 of 50

THEORY OF OPERATION

CONVERTER DETAILS

The AD7616 is a data acquisition system that employs a high

speed, low power, charge redistribution, SAR analog-to-digital

converter (ADC), and allows dual simultaneous sampling of

16 analog input channels. The analog inputs on the AD7616 can

accept true bipolar input signals. Analog input range options

include ±10 V, ±5 V, and ±2.5 V. The AD7616 operates from a

single 5 V supply.

The AD7616 contains input clamp protection, input signal

scaling amplifiers, a first-order antialiasing filter, an on-chip

reference, a reference buffer, a dual high speed ADC, a digital

filter, a flexible sequencer, and high speed parallel and serial

interfaces.

The AD7616 can be operated in hardware or software mode by

controlling the HW_RNGSELx pins. In hardware mode, the

AD7616 is configured by pin control. In software mode, the

AD7616 is configured by the control registers accessed via the

serial or parallel interface.

ANALOG INPUT

Analog Input Channel Selection

The AD7616 contains dual, simultaneous sampling, 16-bit

ADCs. Each ADC has eight analog input channels for a total of

16 analog inputs. Additionally, the AD7616 has on-chip diagnostic

channels to monitor the VCC supply and an on-chip adjustable

low dropout regulator. Channels can be selected for conversion

by control of the CHSELx pins in hardware mode or via the

channel register control in software mode. Software mode is

required to sample the diagnostic channels. Channels can be

selected dynamically or the AD7616 has an on-chip sequencer to

allow the channels for conversion to be preprogrammed. In hard-

ware mode, simultaneous sampling is limited to the corresponding

A and B channel, that is, Channel V0A is always sampled with

Channel V0B. In software mode, it is possible to select any A

channel with any B channel for simultaneous sampling.

Analog Input Ranges

The AD7616 can handle true bipolar, single-ended input

voltages. The logic levels on the range select pins, HW_RNGSEL0

and HW_RNGSEL1, determine the analog input range of all

analog input channels. If both range select pins are tied to a

logic low, the analog input range is determined in software mode

via the input range registers (see the Register Summary section

for more details). In software mode, it is possible to configure

an individual analog input range per channel.

Table 8. Analog Input Range Selection

Analog Input Range

HW_RNGSEL1

HW_RNGSEL0

Configured via the Input

Range Registers

±2.5 V

±5 V

±10 V

In hardware mode, a logic change on these pins has an immediate

effect on the analog input range; however, there is typically a

settling time of approximately 120 µs in addition to the normal

acquisition time requirement. The recommended practice is to

hardwire the range select pins according to the desired input

range for the system signals.

Analog Input Impedance

The analog input impedance of the AD7616 is 1 MΩ, a fixed

input impedance that does not vary with the AD7616 sampling

frequency. This high analog input impedance eliminates the need

for a driver amplifier in front of the AD7616, allowing direct

connection to the source or sensor.

Analog Input Clamp Protection

Figure 41 shows the analog input circuitry of the AD7616. Each

analog input of the AD7616 contains clamp protection circuitry.

Despite single 5 V supply operation, this analog input clamp

protection allows an input overvoltage of between −20 V and

+20 V.

Figure 41. Analog Input Circuitry

Figure 42 shows the input clamp current vs. source voltage

characteristic of the clamp circuit. For source voltages between

−20 V and +20 V, no current flows in the clamp circuit. For input

voltages that are greater than +20 V and less than −20 V, the

AD7616 clamp circuitry turns on.

Figure 42. Input Protection Clamp Profile, Input Clamp Current vs. Source Voltage

Place a series resistor on the analog input channels to limit the

current to ±10 mA for input voltages greater than +20 V and less

than −20 V. In an application where there is a series resistance

on an analog input channel, VxA or VxB, a corresponding

resistance is required on the analog input ground channel,

VxAGND or VxBGND (see Figure 43). If there is no correspond-

1MΩ

CLAMP

Vxx

1MΩ

CLAMP

VxxGND

FIRST-

ORDER

LPF

RFB

–0.25

–0.20

–0.15

–0.10

–0.05

0.05

0.10

0.15

0.20

0.25

–30

–20

–10

SOURCE VOLTAGE (V)

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