AD8566ARMZ Analog Devices Inc, AD8566ARMZ Datasheet - Page 8

• Manufacturer Part Number: AD8566ARMZ
• Description: AMP SENSOR PROG GAIN, 8566, MSOP8
• Manufacturer: Analog Devices Inc

Specifications of AD8566ARMZ

• Op Amp Type: Rail To Rail
• No. Of Amplifiers: 2
• Bandwidth: 5MHz
• Slew Rate: 6V/µs
• Supply Voltage Range: 4.5V To 16V
• Amplifier Case Style: MSOP
• No. Of Pins: 8
• Operating Temperature Range: -40°C To
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD8565/AD8566/AD8567
APPLICATIONS
Theory of Operation
The AD856x family is designed to drive large capacitive loads in
LCD applications. It has high output current drive, rail-to-rail
input/output operation, and is powered from a single 16 V supply.
It is also intended for other applications where low distortion and
high output current drive are needed.
Figure 1 illustrates a simplified equivalent circuit for the AD856x.
The rail-to-rail bipolar input stage is composed of two PNP
differential pairs, Q4 to Q5 and Q10 to Q11, operating in series
with diode protection networks, D1 to D2. Diode network
D1 to D2 serves as protection against large transients for
Q4 to Q5 to accommodate rail-to-rail input swing. D5 to D6
protect Q10 to Q11 against Zenering. In normal operation,
Q10 to Q11 are off and their input stage is buffered from the
operational amplifier inputs by Q6 to D3 and Q8 to D4. Opera-
tion of the input stage is best understood as a function of applied
common-mode voltage: when the inputs of the AD856x are
biased midway between the supplies, the differential signal
path gain is controlled by resistive loads (via R9, R10) Q4 to Q5.
As the input common-mode level is reduced toward the negative
supply (V
and I2, are forced into saturation, thereby forcing the Q6 to D3
and Q8 to D4 networks into cutoff. However, Q4 to Q5 remain
active, providing input stage gain. Inversely, when common-mode
input voltage is increased toward the positive supply, Q4 to Q5
are driven into cutoff, Q3 is driven into saturation, and Q4
becomes active, providing bias to the Q10 to Q11 differential
pair. The point at which Q10 to Q11 differential pair becomes
active is approximately equal to (V
V+
Q6
NEG
Figure 1. AD856x Equivalent Input Circuit
D3
I1
or GND), the input transistor current sources, I1
Q4
R3
R9
D1
Q3
R1
C1
Q10
R5
V
V
D6
POS
C2
D5
NEG
POS
R6
Q11
– 1 V).
Q4
R10
D2
R4
Q5
D4
I2
Q8
BIAS LINE
V–
FOLDED
CASCADE
–8–
The benefit of this type of input stage is low bias current. The
input bias current is the sum of base currents of Q4 to Q5 and
Q6 to Q8 over the range from (V
side of this range, input bias current is dominated by the sum of
base currents of Q10 to Q11 for input signals close to V
Q6 to Q8 (Q10 to Q11) for signals close to V
of design, the input bias current of AD856x not only exhibits
different amplitude but also exhibits different polarities. Figure 2
provides the characteristics of the input bias current versus the
common-mode voltage. It is important to keep in mind that the
source impedances driving the AD856x inputs are balanced for
optimum dc and ac performance.
In order to achieve rail-to-rail output performance, the AD856x
design uses a complementary common-source (or gmRL) output.
This configuration allows output voltages to approach the power
supply rails, particularly if the output transistors are allowed to
enter the triode region on extremes of signal swing, which are
limited by V
Also, this type of output stage exhibits voltage gain in an open-loop
gain configuration. The amount of gain depends on the total
load resistance at the output of the AD856x.
Input Overvoltage Protection
As with any semiconductor device, whenever the input exceeds
either supply voltages, attention needs to be paid to the input
overvoltage characteristics. As an overvoltage occurs, the amplifier
could be damaged, depending on the voltage level and the magnitude
of the fault current. When the input voltage exceeds either supply
by more than 0.6 V, internal pn junctions allow current to flow
from the input to the supplies.
This input current is not inherently damaging to the device as
long as it is limited to 5 mA or less. If a condition exists using
the AD856x where the input exceeds the supply more than 0.6 V,
an external series resistor should be added. The size of the resis-
tor can be calculated by using the maximum overvoltage divided
by 5 mA. This resistance should be placed in series with either
input exposed to an overvoltage.
Figure 2. AD856x Input Bias Current vs. Common-Mode
Voltage
–1,000
1,000
–200
–400
–600
–800
800
600
400
200
0
0
GS
V
T
, the transistor sizes, and output load current.
A
S
= 16V
= 25 C
2
INPUT COMMON-MODE VOLTAGE (V)
4
6
NEG
8
+ 1 V) to (V
10
POS
12
POS
. From this type
– 1 V). Out-
14
NEG
REV. C
16
and of