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AD7606/AD7606-6/AD7606-4

Data Sheet

Rev. D | Page 22 of 36

THEORY OF OPERATION

CONVERTER DETAILS

The AD7606/AD7606-6/AD7606-4 are data acquisition systems

that employ a high speed, low power, charge redistribution,

successive approximation analog-to-digital converter (ADC)

and allow the simultaneous sampling of eight/six/four analog input

channels. The analog inputs on the AD7606/AD7606-6/AD7606-4

can accept true bipolar input signals. The RANGE pin is used to

select either ±10 V or ±5 V as the input range. The AD7606/

AD7606-6/AD7606-4 operate from a single 5 V supply.

The AD7606/AD7606-6/AD7606-4 contain input clamp

protection, input signal scaling amplifiers, a second-order anti-

aliasing 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 AD7606/

AD7606-6/AD7606-4 is controlled using the CONVST signals.

ANALOG INPUT

Analog Input Ranges

The AD7606/AD7606-6/AD7606-4 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 this pin has an

immediate effect on the analog input range; however, 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. 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 the conditions mentioned here may degrade the bipolar zero

code error and THD performance of the AD7606/AD7606-6/

AD7606-4.

Analog Input Impedance

The analog input impedance of the AD7606/AD7606-6/

AD7606-4 is 1 MΩ. This is a fixed input impedance that does

not vary with the AD7606 sampling frequency. This high analog

input impedance eliminates the need for a driver amplifier in

front of the AD7606/AD7606-6/AD7606-4, 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 34 shows the analog input structure of the AD7606/

AD7606-6/AD7606-4. Each analog input of the AD7606/

AD7606-6/AD7606-4 contains clamp protection circuitry.

Despite single 5 V supply operation, this analog input clamp

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

1MΩ

CLAMP

1MΩ

CLAMP

VxGND

SECOND-

ORDER

LPF

RFB

Figure 34. Analog Input Circuitry

Figure 35 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 AD7606/AD7606-6/AD7606-4 clamp circuitry turns on.

–50

–40

–30

–20

–10

–20

–15

–10

–5

SOURCE VOLTAGE (V)

AVCC, VDRIVE = 5V

TA = 25°C

Figure 35. 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 36). If there is

no corresponding resistor on the VxGND channel, an offset

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

overvoltage clamp protection circuitry be used to protect the

AD7606/AD7606-6/AD7606-4 against transient overvoltage

events. It is not recommended to leave the AD7606/AD7606-6/

AD7606-4 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 AD7606/AD7606-6/AD7606-4.

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