ADA4800 데이터 시트보기 (PDF) - Analog Devices

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ADA4800

Low Power, High Speed CCD Buffer Amplifier

Analog Devices 

ADA4800

THEORY OF OPERATION

The ADA4800 is a buffer integrated with an active load. Each

element (the active load and the buffer) operates independently,

as described in the following sections.

SETTING ACTIVE LOAD CURRENT WITH PIN 6 (ISF)

The ISF pin is used to establish the value of the active current

load (IAL). Set the ISF current using Equation 1.

I ISF

=

VISF − 1.55 V

RISF + 3 kΩ

(1)

where:

VISF is referenced to Pin 2. VISF can be an external voltage source,

VCC, or a GPO output as explained in the following paragraphs.

RISF is the external resistor between the ISF pin and VISF.

The active load current (into the IN pin) is directly proportional

to IISF and can be calculated by Equation 2.

IAL = IISF × 27

(2)

The ADA4800 allows for additional power savings by reducing

the active load current. The active load current can be logically

controlled by connecting the ISF pin to any general-purpose

output (GPO) pin of a system microcontroller through an

external resistor. A GPO logic high enables the flow of the

active load current. Appling –VS or connecting a high-Z to the

ISF pin places the ADA4800 into power save mode by shutting

down the active load current.

Figure 22 illustrates an ADA4800 application configuration for

using this power save feature.

An external resistor connected between the ISF and the

microcontroller GPO pin determines the amount of current

that flows into the input pin. This current can be calculated

by using Equation 1 and Equation 2.

SETTING BANDWIDTH WITH PIN 4 (IDRV)

The IDRV pin establishes the buffer’s ICC quiescent current.

As ICC is increased, power dissipation and bandwidth both

increase. Set the current using Equation 3.

I IDRV

=

VIDRV − 0.8 V

RIDRV + 28 kΩ

(3)

where:

VIDRV is referenced to Pin 2. VIDRV can be an external voltage

source or VCC.

RIDRV is the external resistor between the IDRV pin and VIDRV.

The ICC current is directly proportional to IIDRV and can be

calculated by Equation 4.

ICC = IIDRV × 26

(4)

Applying –VS to the IDRV pin shuts down the buffer.

Rev. A | Page 10 of 16

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