Data Sheet

ADA4432-1/ADA4433-1

Rev. E | Page 15 of 25

THEORY OF OPERATION

The ADA4432-1 and ADA4433-1 with short-to-battery and

short-to-ground protection are designed as fifth-order, low-pass

filters with a fixed gain of 2 that is capable of driving 2 V p-p video

signals into doubly terminated video transmission lines on a single

supply as low as 2.6 V. The filter has a 1 dB flatness of 9 MHz

and provides a typical out-of-band rejection of 45 dB at 27 MHz.

The ADA4432-1 is a single-ended filter/driver that can be used

with both ac- and dc-coupled inputs and outputs, with an input

range that includes ground for use with a ground referenced

digital-to-analog converter (DAC) in a single-supply application.

To ensure accurate reproduction of ground referenced signals

without saturating the output devices, an internal offset is

added to shift the output voltage up by 200 mV. For the ac-

coupled input configuration, a dc bias network is needed at the

input of the ADA4432-1. This network can be implemented with

a simple voltage divider between the ac-coupling capacitor and

the input of the ADA4432-1. It is important to remember to

select R and C values appropriate for the frequencies of interest.

The dc bias voltage set point must be well within the input

voltage common-mode range of the ADA4432-1, to

accommodate the full amplitude of the input signal.

The ADA4433-1 is a fully differential filter/driver that is also

designed for compliance with both ac- and dc-coupled inputs and

outputs. The ADA4433-1 can be driven by a differential or single-

ended source and provides a fully differential output signal that

the device is used with a single-ended input source, bias the

inverting input, −IN, at the middle of the input voltage range

applied to the noninverting input, +IN, allowing each output

signal to swing equally around the midsupply point (see the

Configuring the ADA4433-1 for Single-Ended Input Signals

section). This is particularly important to maximize output

voltage headroom in low supply voltage applications.

SHORT CIRCUIT (SHORT-TO-GROUND) PROTECTION

Both the ADA4432-1 and ADA4433-1 include internal protection

circuits that limit the output sink or source current to 60 mA.

This short circuit protection prevents damage to the ADA4432-1

and ADA4433-1 when the output(s) are shorted to ground, to a

low impedance source, or together (in the case of the ADA4433-1)

for an extended time. In addition, in the case of the ADA4433-1,

the total sink or source current for both outputs is limited to

50 mA, which helps protect the device in the event of both outputs

being shorted to a low impedance. However, short circuit

protection does not affect the normal operation of the devices

because one output sources current, whereas the other output

sinks current when driving a differential output signal.

OVERVOLTAGE (SHORT-TO-BATTERY) PROTECTION

Both the ADA4432-1 and ADA4433-1 include internal protection

circuits to ensure that internal circuitry is not subjected to

extreme voltages or currents during an overvoltage event

applied to their outputs. A short-to-battery condition usually

consists of a voltage on the outputs that is significantly higher

than the power supply voltage of the amplifier. Duration can

vary from a short transient to a continuous fault.

The ADA4432-1 and ADA4433-1 can withstand voltages of up

to 18 V on the outputs. Critical internal nodes are protected from

exposure to high voltages by circuitry that isolates the output

devices from the high voltage and limits internal currents. This

protection is available whether the device is enabled or disabled,

even when the supply voltage is removed.

The output devices are disconnected when the voltage at the

output pins exceeds the supply voltage. After the overvoltage

condition is removed, internal circuitry pulls the output voltage

back within normal operating levels. The output devices are

reconnected when the voltage at the output pins falls below the

supply voltage by about 300 mV. When the devices are used with a

doubly terminated cable, the voltage sensed at the output pins is

lower than the voltage applied to the cable by the voltage drop

across the back termination resistor. The maximum voltage drop

across the back termination resistor is limited by the short-circuit

current protection; therefore, the threshold at which the over-

voltage protection responds to a voltage applied to the cable is

where:

internal isolation circuitry.

voltage seen at the output pins is lower than the supply voltage,

so no overvoltage condition is detected. However, the internal

circuitry is protected by the short circuit current limit; therefore,

the ADA4432-1/ADA4433-1 can withstand an indefinite duration

short to any positive voltage up to 18 V without damage.

SHORT-TO-BATTERY OUTPUT FLAG

In addition to the internal protection circuitry, the short-to-

battery output flag (STB pin) indicates an overvoltage condition

on either or both output pins. The flag is present whenever the

internal overvoltage protection is active; therefore, it is available

when the device is enabled or disabled. It is not available, however,

when the supply voltage is removed, although the internal

protection is still active. The threshold at which the short-to-

battery flag is activated and deactivated is the same as the

threshold for the protection circuitry.

Table 8. STB Pin Logic

STB Pin Output

Device State

High (Logic 1)

Overvoltage fault condition

Low (Logic 0)

Normal operation