AD7608BSTZ datasheet(19/32 Pages) AD

19 / 32 page

Data Sheet

AD7608

Rev. A | Page 19 of 32

THEORY OF OPERATION

CONVERTER DETAILS

The AD7608 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 AD7608 can accept true bipolar input

signals. The RANGE pin is used to select either ±10 V or

±5 V as the input range. The AD7608 operates from a single

5 V supply.

The AD7608 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 AD7608 is controlled using the

CONVST x signals.

ANALOG INPUT

Analog Input Ranges

The AD7608 can handle true bipolar, single-ended 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 the RANGE pin has an immediate

effect on the analog input range; how-ever, there is typically

a settling time of approximately 80 µs, in addition to the normal

acquisition time requirement. The 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 AD7608.

Analog Input Impedance

The analog input impedance of the AD7608 is 1 MΩ. This is

a fixed input impedance that does not vary with the AD7608

sampling frequency. This high analog input impedance elimi-

nates the need for a driver amplifier in front of the AD7608,

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 31 shows the analog input structure of the AD7608. Each

AD7608 analog input contains clamp protection circuitry. Despite

single 5 V supply operation, this analog input clamp protection

allows for an input overvoltage up to ±16.5 V.

1MΩ

CLAMP

1MΩ

CLAMP

VxGND

SECOND-

ORDER

LPF

RFB

Figure 31. Analog Input Circuitry

Figure 32 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 AD7608 clamp circuitry turns on.

–40

–30

–20

–10

–25

–20

–15

–10

–5

SOURCE VOLTAGE (V)

AVCC, VDRIVE = 5V

TA = 25 °C

Figure 32. 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 33). If there is

no corresponding resistor on the VxGND channel, an offset

error occurs on that channel.

1MΩ

CLAMP

1MΩ

CLAMP

VxGND

RFB

ANALOG

INPUT

SIGNAL

AD7608

Figure 33. Input Resistance Matching on the Analog Input

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