ADA4945-1ACPZ-R2 datasheet(36/44 Pages) AD

36 / 44 page

ADA4945-1

Data Sheet

Rev. 0 | Page 36 of 44

OUTPUT VOLTAGE CLAMP

In addition to the differential and common-mode signal paths,

the ADA4945-1 implements clamping circuits to protect the

input devices of circuits being driven by the ADA4945-1,

hereafter assumed to be an ADC, from being overdriven and

potentially damaged. These clamping circuits use both

differential and common-mode feedback to limit the output

voltages to a range defined by the voltage applied to two

reference pins, +VCLAMP and −VCLAMP. These high impedance

pins are typically connected to potentials that define the

allowable input range of the ADC, which are the ADC reference

voltages (+VREF and −VREF) for most ADCs.

As shown in Figure 100, the common-mode clamping circuit

senses the output voltage midpoint and applies a common-

mode feedback signal to prevent VOUT, cm from exceeding

+VCLAMP or going below −VCLAMP.

–OUT

+VCLAMP

UPPER

COMMON-MODE

CLAMP

LOWER

COMMON-MODE

CLAMP

iCLAMP (CM)

–VCLAMP

+OUT

Figure 100. Common-Mode Clamp Block Diagram

The differential clamping circuit, shown in Figure 101, senses

each output (+OUT and −OUT) and applies a differential

feedback signal to prevent either output from exceeding

(+VCLAMP + 0.5 V) or going below (−VCLAMP − 0.5 V). The

approximately 500 mV offset voltage is designed to allow the

outputs to fully use the input range of the ADC without any

clamp engagement, while providing input protection prior to

the turn on of the ADC input protection diodes. This feature

allows the ADA4945-1 to provide a full-scale signal to the ADC

without incurring any clamp induced distortion, thus

maximizing signal-to-noise ratio (SNR) and linearity while

protecting the ADC inputs.

–OUT

+VCLAMP

UPPER

DIFFERENTIAL

CLAMP

iCLAMP (DIFF)

+OUT

–

500 mV

LOWER

DIFFERENTIAL

CLAMP

–VCLAMP

500 mV

iCLAMP (DIFF)

Figure 101. Differential Clamp Block Diagram

By applying a differential feedback signal in response to one or

both outputs exceeding the clamp reference voltages, both

outputs are limited equally, even if only one output exceeds one

of the clamp reference voltages. This feature allows the ADA4945-

1 to maintain a constant output common-mode voltage even

while clamping the differential outputs, which enables a faster

system recovery from a clamped condition.

In systems where output clamping is not desired, the upper

output clamp can be disabled by connecting +VCLAMP to +VS,

and the lower output clamp can be disabled by connecting

−VCLAMP to −VS. If one clamp is disabled (for example, −VS =

−VCLAMP = 0 V), the other can be remain active, and the output

is limited when either or both outputs reaches the active clamp

reference.

An additional feature of the ADA4945-1 is the use of a resistor

divider between the +VCLAMP and −VCLAMP pins, as shown in

Figure 99, to set the default potential on the VOCM pin when the

pin is not externally driven. Because the +VCLAMP and −VCLAMP

pins are typically set to the maximum and minimum desired

input voltage of the ADC (for example, +VREF and −VREF),

respectively, this resistor divider sets the output common-mode

voltage of the ADA4945-1 at the midpoint of the ADC input

range by default. By contrast, most fully-differential amplifiers

use a resistor divider between the amplifier supply voltages to

set the default output common-mode voltage, which may not be

optimal for maximizing ADC input range usage.

POWER MODES

The ADA4945-1 implements two fully characterized active

power modes (full power, low power) and a disable mode to

optimize system power and performance trade-offs. The

transition time from disable mode to either of the active power

modes is fast (<2 μs), allowing additional power savings by

dynamically placing the ADA4945-1 in disable mode when the

output voltage is not needed (for example, between ADC

samples in low data rate systems).

ADA4945-1ACPZ-R2 датащи(PDF) 36 Page - Analog Devices

ADA4945-1ACPZ-R2 датащи(HTML) 36 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

датащи скачать

ссылки URL