AD4697 datasheet(33/107 Pages) AD

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Data Sheet

AD4697/AD4698

THEORY OF OPERATION

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Rev. 0 | 33 of 107

Figure 73. Amplifier and RC Filter Performance vs. Analog Input High-Z Mode Test Circuit

INPUT OVERVOLTAGE PROTECTION CLAMPS

The AD4697/AD4698 include overvoltage protection clamps on

IN0 to IN7 and COM to reduce the risk of device damage from

sustained dc overvoltage events. These clamps eliminate the need

for external clamping diodes in systems where the input driving

circuitry positive supply rail is greater than VREF (see Figure 112).

Table 1 shows the activation, deactivation, and clamping voltages of

the overvoltage protection clamps. Figure 36 and Figure 39 show

the typical behavior of the clamps during overvoltage conditions.

The clamp circuits activate when the analog input voltage exceeds

the activation voltage. The clamps deactivate when the input volt-

age drops below the deactivation voltage. While a clamp is active, a

flag is set in the status registers that can be read by the digital host.

See the Overvoltage Clamp Flags section for a detailed description

of the options for reading the status of each clamp.

The overvoltage protection clamps limit the extent to which input

overvoltage events disturb the reference source. When active, the

clamps limit the voltage on the analog inputs to the specified

clamping voltage and conduct the input current to ground rather

than through the ESD diode connecting the analog input to the

REF input (D1 in Figure 66), which prevents overvoltage conditions

on one analog input from degrading performance on other analog

inputs or other devices sharing the reference. Figure 43 shows the

relationship between a single clamp input current and the resulting

additional reference input current.

Figure 24, Figure 27, and Figure 31 through Figure 33 show

the offset error, gain error, and ac performance for one analog

input channel vs. the total number of active overvoltage protection

clamps on the other inputs.

Each overvoltage protection clamp circuit supports a maximum

sustained current of 5 mA. All nine clamp circuits can sink 5 mA

simultaneously without damaging the device. The clamp current is

a function of VREF, the external series resistance (such as REXT

in Figure 66), and the output voltage of the AFE circuitry. See the

Input Overvoltage Protection Clamps section for details on how to

select REXT to prevent excess clamp current during overvoltage

events.

Overvoltage Reduced Current Mode

The overvoltage reduced current mode further reduces the addition-

al reference current during overvoltage events. Figure 43 shows the

difference between the additional reference input currents drawn

for the different clamp input currents with the overvoltage reduced

current mode enabled and disabled.

The overvoltage reduced current mode is enabled when the

OV_MODE bit in the REF_CTRL register is set to 0. Overvoltage

reduced current mode is enabled by default.

Enabling overvoltage reduced current mode changes the maximum

value of REXT and achieves stable clamp operation. See the Over-

voltage Protection Clamp Stability section for more information on

the relationship between the external RC filter and clamp operation.

Overvoltage Protection Clamp Stability

In applications where analog input overvoltage events are not a

concern, or in applications where clamp stability is not a concern,

the REXT and CEXT values are not required to follow the guidelines

described in this section.

The stability of the overvoltage protection clamp circuits depends

on the external RC filter component values and whether the over-

voltage reduced current mode is enabled or disabled. When a

clamp is unstable, it toggles between the active and inactive states

during overvoltage events. This instability causes small, modulating

currents to flow in both the overdriven input and the reference,

which can result in measurement errors in the conversions of other

analog inputs if the reference circuitry does not have adequate load

regulation to maintain a stable reference voltage in response to

the additional reference current. Table 1 and Figure 43 show the

additional reference input (REF) current per active clamp.

To ensure stable clamp operation, CEXT in the external RC filter

(as shown in Figure 66) must be at least 500 pF. The maximum

value of REXT is 1 kΩ when the overvoltage reduced current mode

is enabled, and 2 kΩ when the overvoltage reduced current mode is

disabled.

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