AD7607 datasheet(19/32 Pages) AD

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Data Sheet

AD7607

Rev. B | Page 19 of 32

THEORY OF OPERATION

CONVERTER DETAILS

The AD7607 is a data acquisition system that employs a high

speed, low power, charge redistribution, successive approxi-

mation analog-to-digital converter (ADC) and allows the

simultaneous sampling of eight analog input channels. The analog

inputs on the AD7607 can accept true bipolar input signals. The

RANGE pin is used to select either ±10 V or ±5 V as the input

range. The AD7607 operates from a single 5 V supply.

The AD7607 contains input clamp protection, input signal scaling

amplifiers, a second-order antialiasing filter, track-and-hold

amplifiers, an on-chip reference, reference buffers, a high speed

ADC, a digital filter, and high speed parallel and serial interfaces.

Sampling on the AD7607 is controlled using the CONVST signals.

ANALOG INPUT

Analog Input Ranges

The AD7607 can handle true bipolar input voltages. The logic

level on the RANGE pin determines the analog input range of

all analog input channels. If this pin is tied to a logic high, the

analog input range is ±10 V for all channels. If this pin is tied

to a logic low, the analog input range is ±5 V for all channels.

A logic change on this pin has an immediate effect on the analog

input range; however, there is a typical settling time of ~80 µs,

in addition to the normal acquisition time requirement.

Recommended practice is to hardwire the RANGE pin

according to the desired input range for the system signals.

During normal operation, the applied analog input voltage should

remain within the analog input range selected via the range pin.

A RESET pulse must be applied after power-up to ensure the

analog input channels are configured for the range selected.

When in a power-down mode, it is recommended to tie the

analog inputs to GND. As per the input clamp protection section,

the overvoltage clamp protection is recommended for use in

transient overvoltage conditions and should not remain active

for extended periods. Stressing the analog inputs outside of the

conditions mentioned here may degrade the Bipolar Zero Code

error and THD performance of the AD7607.

Analog Input Impedance

The analog input impedance of the AD7607 is 1 MΩ. This is

a fixed input impedance that does not vary with the AD7607

sampling frequency. This high analog input impedance elimi-

nates the need for a driver amplifier in front of the AD7607,

allowing for direct connection to the source or sensor. With the

need for a driver amplifier eliminated, bipolar supplies (which

are often a source of noise in a system) can be removed from

the signal chain.

Analog Input Clamp Protection

Figure 30 shows the analog input structure of the AD7607.

Each AD7607 analog input contains clamp protection circuitry.

Despite single 5 V supply operation, this analog input clamp

protection allows for an input overvoltage of up to ±16.5 V.

1MΩ

CLAMP

1MΩ

CLAMP

VxGND

SECOND-

ORDER

LPF

RFB

Figure 30. Analog Input Circuitry

Figure 31 shows the voltage vs. current characteristic of the

clamp circuit. For input voltages of up to ±16.5 V, no current

flows in the clamp circuit. For input voltages that are above ±16.5 V,

the AD7607 clamp circuitry turns on and clamps the analog

input to ±16.5 V.

–50

–40

–30

–20

–10

–20

–15

–10

–5

SOURCE VOLTAGE (V)

AVCC, VDRIVE = 5V

TA = 25°C

Figure 31. Input Protection Clamp Profile

A series resistor should be placed on the analog input channels

to limit the current to ±10 mA for input voltages above ±16.5 V.

In an application where there is a series resistance on an analog

input channel, Vx, a corresponding resistance is required on the

analog input GND channel, VxGND (see Figure 32). If there is

no corresponding resistor on the VxGND channel, an offset

error occurs on that channel.

1MΩ

CLAMP

VINx

1MΩ

CLAMP

VxGND

RFB

ANALOG

INPUT

SIGNAL

AD7607

Figure 32. Input Resistance Matching on the Analog Input

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