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Data Sheet

AD7609

Rev. A | Page 21 of 36

THEORY OF OPERATION

CONVERTER DETAILS

The AD7609 is a data acquisition system that employs a high

speed, low power, charge redistribution successive approxima-

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

simultaneous sampling of eight true differential analog input

channels. The analog inputs on the AD7609 can accept true

bipolar input signals. The RANGE pin is used to select either

±10 V or ±5 V as the input range. The AD7609 operates from

a single 5 V supply.

The AD7609 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 AD7609 is controlled using

CONVST x signals.

ANALOG INPUT

Analog Input Ranges

The AD7609 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 settling time of 80 µs

typically, 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 to the part 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 together or both analog input pins (Vx+, Vx−) to

GND. As per the Analog 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

these conditions may degrade the Bipolar Zero Code error and

THD performance of the AD7609.

Analog Input Impedance

The analog input impedance of the AD7609 is 1 MΩ. This is a

fixed input impedance and does not vary with the AD7609 sam-

pling frequency. This high analog input impedance eliminates

the need for a driver amplifier in front of the AD7609 allowing

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

a driver amplifier eliminated, bipolar supplies can be removed

from the signal chain, which are often a source of noise in a system.

Analog Input Clamp Protection

Figure 32 shows the analog input structure of the AD7609.

Each AD7609 analog input contains clamp protection circuitry.

Despite a single 5 V supply operation, this analog input clamp

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

1MΩ

CLAMP

Vx+

1MΩ

CLAMP

Vx–

SECOND-

ORDER

LPF

RFB

Figure 32. Analog Input Circuitry

Figure 33 shows the current vs. voltage characteristic of the

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

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

AD7609 clamp circuitry turns on and clamps the analog input

to ±16.5 V. A series resister 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, VINx+, a corresponding resistance

is required on the VINx− channel (see Figure 34). If there is no

corresponding resister on the Vx− channel, this results in an

offset error on that channel. It is recommended that the input

overvoltage clamp protection circuitry be used to protect the

AD7609 against transient overvoltage events. It is not recom-

mended to leave the AD7609 in a condition where the clamp

protection circuitry is active (in normal or power-down

conditions) for extended periods because this may degrade the

bipolar zero code error performance of the AD7609.

–50

–40

–30

–20

–10

–20

–15

–10

–5

SOURCE VOLTAGE (V)

AVCC, VDRIVE = 5V

TA = 25°C

Figure 33. Input Protection Clamp Profile

1MΩ

CLAMP

VINx+

1MΩ

CLAMP

VINx–

RFB

+10V

–10V

AD7609

+10V

–10V

Figure 34. Input Resistance Matching on the Analog Input

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