AD8515AKSZ-REEL Datasheet AD8515AKSZ-REEL7, AD8515ARTZ-REEL7, AD8515ARTZ-REEL | P13-P15

AD8515 Data Sheet

Rev. E | Page 12 of 16

THEORY OF OPERATION

The AD8515, offered in space-saving SOT-23 and SC70 packages,

is a rail-to-rail input and output operational amplifier that can

operate at supply voltages as low as 1.8 V. This product is fabri-

cated using 0.6 micron CMOS to achieve one of the best power

consumption-to-speed ratios (that is, bandwidth) in the industry.

With a small amount of supply current (less than 400 μA),

a wide unity gain bandwidth of 4.5 MHz is available for signal

processing.

The input stage consists of two parallel, complementary, differ-

ential pairs of PMOS and NMOS. The AD8515 exhibits no

phase reversal because the input signal exceeds the supply by

more than 0.6 V. Currents into the input pin must be limited

to 5 mA or less by the use of external series resistance(s). The

AD8515 has a very robust ESD design and can stand ESD

voltages of up to 4000 V.

POWER CONSUMPTION vs. BANDWIDTH

One of the strongest features of the AD8515 is the bandwidth

stability over the specified temperature range while consuming

small amounts of current. This effect is shown in Figure 2 through

Figure 4.

This product solves the speed/power requirements for many

applications. The wide bandwidth is also stable even when operated

with low supply voltages. Figure 5 shows the relationship between

the supply voltage vs. the bandwidth for the AD8515.

The AD8515 is ideal for battery-powered instrumentation and

handheld devices because it can operate at the end of discharge

voltage of most popular batteries. Table 6 lists the nominal and

end of discharge voltages of several typical batteries.

Table 6. Typical Battery Life Voltage Range

Battery

Nominal

Voltage (V)

End of Discharge

Voltage (V)

Lead-Acid 2 1.8

Lithium 2.6 to 3.6 1.7 to 2.4

NiMH 1.2 1

NiCd 1.2 1

Carbon-Zinc 1.5 1.1

Data Sheet AD8515

Rev. E | Page 13 of 16

DRIVING CAPACITIVE LOADS

Most amplifiers have difficulty driving large capacitive loads.

Additionally, higher capacitance at the output can increase the

amount of overshoot and ringing in the amplifier’s step response

and can even affect the stability of the device. This is due to the

degradation of phase margin caused by additional phase lag from

the capacitive load. The value of capacitive load that an amplifier

can drive before oscillation varies with gain, supply voltage, input

signal, temperature, and other parameters. Unity gain is the most

challenging configuration for driving capacitive loads. The AD8515

is capable of driving large capacitive loads without any external

compensation. The graphs in Figure 31 and Figure 32 show the

amplifier’s capacitive load driving capability when configured

in unity gain of +1.

The AD8515 is even capable of driving higher capacitive loads

in inverting gain of −1, as shown in Figure 33.

03024-031

VOLTAGE (100mV/DIV)

TIME (1µs/DIV)

= ±2.5V

= 50pF

GAIN = 1

igure 31. Capacitive Load Driving @ C

= 50 pF

03024-032

VOLTAGE (10mV/DIV)

TIME (1µs/DIV)

= ±2.5V

= 500pF

GAIN = 1

igure 32. Capacitive Load Driving @ C

= 500 pF

03024-033

VOLTAGE (100mV/DIV)

TIME (1µs/DIV)

= ±0.9V

= 800pF

GAIN = –1

igure 33. Capacitive Load Driving @ C

= 800 pF

FULL POWER BANDWIDTH

The slew rate of an amplifier determines the maximum frequency

at which it can respond to a large input signal. This frequency

(known as full power bandwidth, FPBW) can be calculated from

the equation

PEAK

FPBW

×π

for a given distortion. The FPBW of the AD8515 is shown in

Figure 34 to be close to 200 kHz.

03024-034

VOLTAGE (2V/DIV)

TIME (2µs/DIV)

OUT

igure 34. Full Power Bandwidth

AD8515 Data Sheet

Rev. E | Page 14 of 16

A MICROPOWER REFERENCE VOLTAGE GENERATOR

Many single-supply circuits are configured with the circuit

biased to one-half of the supply voltage. In these cases, a false

ground reference can be created by using a voltage divider buffered

by an amplifier. Figure 35 shows the schematic for such a circuit.

The two 1 MΩ resistors generate the reference voltages while

drawing only 0.9 μA of current from a 1.8 V supply. A capacitor

connected from the inverting terminal to the output of the op amp

provides compensation to allow for a bypass capacitor to be

connected at the reference output. This bypass capacitor helps

establish an ac ground for the reference output.

03024-035

AD8515

1µF

0.022µF

1µF

10kΩ

1MΩ

100Ω

1.8V TO 5V

0.9V TO 2.5V

1MΩ

V+

V–

–

igure 35. Micropower Voltage Reference Generator

A 100 kHz SINGLE-SUPPLY SECOND-ORDER

BAND-PASS FILTER

The circuit in Figure 36 is commonly used in portable applica-

tions where low power consumption and wide bandwidth are

required. This figure shows a circuit for a single-supply band-pass

filter with a center frequency of 100 kHz. It is essential that the

op amp have a loop gain at 100 kHz to maintain an accurate center

frequency. This loop gain requirement necessitates the choice of

an op amp with a high unity gain crossover frequency, such as

the AD8515. The 4.5 MHz bandwidth of the AD8515 is sufficient

to accurately produce the 100 kHz center frequency, as the response

in Figure 37 shows. When the op amp bandwidth is close to the

center frequency of the filter, the amplifier internal phase shift

causes excess phase shift at 100 kHz, altering the filter response.

In fact, if the chosen op amp has a bandwidth close to 100 kHz,

the phase shift of the op amps causes the loop to oscillate.

A common-mode bias level is easily created by connecting the

noninverting input to a resistor divider consisting of two resistors

connected between VCC and ground. This bias point is also

decoupled to ground with a 1 μF capacitor.

C1R1

××π

C1R1

××π

H += 1

V5V8.1 −=VCC

here:

is the low −3 dB frequency.

is the high −3 dB frequency.

is the midfrequency gain.

03024-036

400mV

V11

VCC

VOUT

1MΩ

1µF

2nF

5kΩ

20kΩ

2kΩ

AD8515

V+

V–

–

10pF

igure 36. Second-Order Band-Pass Filter

1k 10k 100M

03024-037

OUTPUT VOLTAGE (V)

FREQUENCY (Hz)

100k 1M 10M

1.5

0.5

igure 37. Frequency Response of the Band-Pass Filter

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P17

AD8515AKSZ-REEL

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Manufacturer:

Analog Devices Inc.

Description:

Operational Amplifiers - Op Amps 1.8V Lo Pwr CMOS RRIO

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