AD7617BSTZ datasheet(24/52 Pages) AD

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Data Sheet

AD7617

Rev. 0 | Page 23 of 51

THEORY OF OPERATION

CONVERTER DETAILS

The AD7617 is a data acquisition system that employs a high

speed, low power, charge redistribution, SAR ADC, and allows

dual simultaneous sampling of 16 analog input channels. The

analog inputs on the AD7617 can accept true bipolar input

signals. Analog input range options include ±10 V, ±5 V, and

±2.5 V. The AD7617 operates from a single 5 V supply.

The AD7617 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 AD7617 can operate in hardware or software mode by

controlling the HW_RNGSELx pins. In hardware mode, the

AD7617 is configured by pin control. In software mode, the

AD7617 is configured by the control registers accessed via the

serial or parallel interface.

ANALOG INPUT

Analog Input Channel Selection

The AD7617 contains dual, simultaneous sampling, 14-bit ADCs.

Each ADC has eight analog input channels for a total of 16 analog

inputs. Additionally, the AD7617 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 AD7617 has an on-chip sequencer to allow the channels

for conversion to be preprogrammed. In hardware mode,

simultaneous sampling is limited to the corresponding A or B

channel, that is, Channel V0A always samples 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 AD7617 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 low drift analog input impedance of the AD7617 is 1 MΩ,

a fixed input impedance that does not vary with the AD7617

sampling frequency. This high analog input impedance eliminates

the need for a driver amplifier in front of the AD7617, allowing

direct connection to the source or the sensor.

Analog Input Clamp Protection

Figure 42 shows the analog input circuitry of the AD7617. Each

analog input of the AD7617 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 42. Analog Input Circuitry

Figure 43 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

AD7617 clamp circuitry turns on.

Figure 43. 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 44).

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)

POWERED OFF

POWERED ON

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