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ADG467BRSZ

Octal Channel Protectors

Analog Devices 

ADG467

When a negative overvoltage is applied to the channel protector

circuit, the PMOS transistor enters a saturated mode of operation

as the drain voltage exceeds VSS − VTP (see Figure 24). As in the

case of the positive overvoltage, the other MOS devices are

nonsaturated.

NEGATIVE

OVERVOLTAGE

(–20V)

VSS – VTP1

(–13.5V)

NEGATIVE

NMOS

OVERVOLTAGE

(–20V)

NON-

SATURATED

PMOS

SATURATED

NMOS

NON-

SATURATED

VDD (+15V)

VSS (–15V)

VDD (+15V)

1VTP = PMOS THRESHOLD VOLTAGE (–1.5V).

Figure 24. Negative Overvoltage on the Channel Protector

The channel protector is also functional when the supply rails

are down (for example, power failure) or momentarily uncon-

nected (for example, rack system). This is where the channel

protector has an advantage over more conventional protection

methods such as diode clamping (see the Applications Information

section). When VDD and VSS equal 0 V, all transistors are off and

the current is limited to subnano-ampere levels (see Figure 25).

(0V)

POSITIVE OR

NEGATIVE

OVERVOLTAGE

NMOS

OFF

PMOS

OFF

NMOS

OFF

VDD (0V)

VSS (0V)

VDD (0V)

Figure 25. Channel Protector Supplies Equal to 0 V

Rev. B | Page 10 of 16

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