AD8026

–6–

REV. 0

FREQUENCY – Hz

100

10k

100M

100k

1M

10M

0.1

0.01

10

1

1G

Figure 15. Output Impedance vs. Frequency

FREQUENCY – Hz

1M

10k

100M

100k

1M

10M

1k

100

10k

1G

100k

10

Figure 16. Input Impedance vs. Frequency

FREQUENCY – Hz

20

–10

–20

10

0

1

0

24.9

49.9

100

3k

1875

+

–

5k

100k

1M

10M

100M

500M

Figure 17. Bandwidth and Flatness vs. Series Resistance

into 100 pF

THEORY OF OPERATION

The AD8026, a quad voltage feedback amplifier with rail-to-rail

output swing, is internally configured for a gain of either –5/3 or

+8/3. The gain-setting resistors are laser trimmed for precise

control of their ratio. In addition, the amplifier’s frequency

response has been adjusted to compensate for the parasitic

capacitances associated with the gain resistors and with the

amplifier’s inverting input. The result is an amplifier with very

tight control of closed-loop gain and settling time.

The amplifier’s input stage will operate with voltages from about

–0.2 V below the negative supply voltage to within about 1 V of

the positive supply. Exceeding these values will not cause phase

reversal at the output; however, the input ESD protection de-

vices will begin to conduct if the input voltages exceed the sup-

ply rails by greater than 0.5 V. The gain resistors that connect to

Pins 2, 6, 9, and 13 are protected from ESD in such a way that

the voltages applied to these pins may exceed the negative sup-

ply by as much as –7 V.

The rail-to-rail output range of the AD8026 is provided by a

complementary common-emitter output stage. The chosen

circuit topology allows the outputs to source and sink 50 mA of

output current and, with the use of an external series resistor, to

achieve rapid settling time while driving capacitive loads within

0.5 V of the supply rails.

Output Referred Offset Voltage

The output referred offset voltage for a voltage feedback ampli-

fier can be estimated with the following equation:

= V

IOS

OS

× R

B

× R

P

− R

()

()

where:

Ω, R

Ω for this part.

This equation leads to the well known conclusion that, for a

voltage feedback amplifier to maintain minimum output offset

was designed for an assumed source impedance, of 500

Ω driv-

is 500

Ω less than the ideal value for minimum output offset.

Additional resistance may be added externally, in series with the

+Input, if the part is to be driven by a lower impedance source.

APPLICATIONS

The AD8026 is designed with on-chip resistors for each op amp

to provide accurate fixed gain and low output-referenced offset

voltages. This can result in significant cost and board-space savings

for systems that can take advantage of the AD8026 specifications.

The part is actually trimmed in three steps. First, the supply

current of the part is trimmed. Then the gain is accurately

trimmed to specification. This trim adjusts the values of either

the gain or feedback resistor for a ratio of 5 to 3. The final trim

is for the offset voltage. For this trim, the –Input is connected to

ground and the +Input is connected to ground via 500

Ω, while

internal offset resistors are trimmed.