AD8546 datasheet(18/24 Pages) AD

18 / 24 page

AD8546

Rev. 0 | Page 18 of 24

OUTPUT STAGE

The AD8546 features a complementary output stage consisting

of the M16 and M17 transistors (see Figure 62). These transistors

are configured in Class AB topology and are biased by the voltage

source, VB2. This topology allows the output voltage to go within

millivolts of the supply rails, achieving a rail-to-rail output swing.

The output voltage is limited by the output impedance of the

transistors, which are low RON MOS devices. The output voltage

swing is a function of the load current and can be estimated using

the output voltage to supply rail vs. load current diagrams (see

Figure 15, Figure 16, Figure 18, and Figure 19).

RAIL-TO-RAIL INPUT AND OUTPUT

The AD8546 features rail-to-rail input and output with a supply

voltage from 2.7 V to 18 V. Figure 63 shows the input and output

waveforms of the AD8546 configured as a unity-gain buffer with

a supply voltage of ±9 V and a resistive load of 1 MΩ. With an

input voltage of ±9 V, the AD8546 allows the output to swing

very close to both rails. Additionally, it does not exhibit phase

reversal.

TIME (200µs/DIV)

VSY = ±9V

RL = 1MΩ

INPUT

OUTPUT

Figure 63. Rail-to-Rail Input and Output

RESISTIVE LOAD

The feedback resistor alters the load resistance that an amplifier

sees. It is, therefore, important to be aware of the value of the

feedback resistors selected for use with the AD8546. The AD8546

is capable of driving resistive loads down to 100 kΩ. The following

two examples, inverting and noninverting configurations, show

how the feedback resistor changes the actual load resistance seen

at the output of the amplifier.

Inverting Configuration

Figure 64 shows the AD8546 in an inverting configuration with a

resistive load, RL, at the output. The actual load seen by the ampli-

fier is the parallel combination of the feedback resistor, R2, and

the load, RL. The combination of a feedback resistor of 1 kΩ and

a load of 1 MΩ results in an equivalent load resistance of 999 Ω at

the output. In this condition, the AD8546 is incapable of driving

such a heavy load; therefore, its performance degrades greatly.

To avoid loading the output, use a larger feedback resistor, but

consider the effect of resistor thermal noise on the overall circuit.

AD8546

1/2

–VSY

RL, EFF = RL || R2

+VSY

VIN

VOUT

Figure 64. Inverting Op Amp Configuration

Noninverting Configuration

Figure 65 shows the AD8546 in a noninverting configuration

with a resistive load, RL, at the output. The actual load seen by

the amplifier is the parallel combination of R1 + R2 and RL.

–VSY

RL, EFF = RL || (R1 + R2)

+VSY

VIN

VOUT

AD8546

1/2

Figure 65. Noninverting Op Amp Configuration

COMPARATOR OPERATION

An op amp is designed to operate in a closed-loop configuration

with feedback from its output to its inverting input. Figure 66

shows the AD8546 configured as a voltage follower with an input

voltage that is always kept at midpoint of the power supplies.

The same configuration is applied to the unused channel. A1 and

A2 indicate the placement of ammeters to measure supply current.

ISY+ refers to the current flowing from the upper supply rail to the

op amp, and ISY− refers to the current flowing from the op amp to

the lower supply rail. As expected, Figure 67 shows that in normal

operating condition, the total current flowing into the op amp is

equivalent to the total current flowing out of the op amp, where

ISY+ = ISY− = 36 μA for the dual AD8546 at VSY = 18 V.

AD8546

1/2

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

Figure 66. Voltage Follower Configuration

AD8546 датащи(PDF) 18 Page - Analog Devices

AD8546 датащи(HTML) 18 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

датащи скачать

ссылки URL