AD6555ABC Analog Devices Inc, AD6555ABC Datasheet - Page 15

• Manufacturer Part Number: AD6555ABC
• Manufacturer: Analog Devices Inc

Specifications of AD6555ABC

• Lead Free Status / Rohs Status: Not Compliant

A second major difference is that the output only sinks 1 mA to
the negative supply. There is no pulldown stage at the output
other than the 1 mA current source used for the V-to-F
conversion. The op amp sources a great deal of current from the
positive supply, and it is internally protected by current limiting.
The output of the op amp can be driven to within 3 V of the
positive supply when it is not sourcing external current. When
sourcing 10 mA the output voltage can be driven to within 6 V
of the positive supply.
A third difference between this op amp and a normal device is
that the inverting input, Pin 3, is bias current compensated and
the noninverting input is not bias-current compensated. The
bias current at the inverting input is nominally zero, but can be
as much as 20 nA in either direction. The noninverting input
typically has a bias current of 40 nA that always flows into the
node (an npn input transistor). Therefore, it is not possible to
match input voltage drops due to bias currents by matching
input resistors.
The op amp has provisions for trimming the input offset
voltage. A potentiometer of 20 kΩ is connected from Pin 13 to
Pin 14 and the wiper is connected to the positive supply
through a 250 kΩ resistor. A potential of about 0.6 V is
established across the 250 kΩ resistor, and the 3 μA current is
injected into the null pins. It is also possible to null the op amp
offset voltage by using only one of the null pins and by using a
bipolar current either into or out of the null pin. The amount of
current required is very small—typically less than 3 μA. This
technique is shown in the Applications section of this data
sheet; the autozero circuit uses this technique.
Rev. D | Page 15 of 20
The bipolar offset current is activated by connecting a 1.24 kΩ
resistor between Pin 4 and the negative supply. The resulting
current delivered to the op amp noninverting input is nominally
0.5 mA and has a tolerance of ±10%. This current is then used
to provide an offset voltage when Pin 2 is tied to ground through
a resistor. The 0.5 mA that appears at Pin 2 is also flowing
through the 1.24 kΩ resistor. An external resistor is used to
activate the bipolar offset current source to provide the lowest
tolerance and temperature drift of the resulting offset voltage.
It is possible to use other values of resistance between Pin 4 and
−V
Figure 19 shows the relationship between the bipolar offset
current and the value of the resistor used to activate the source.
S
to obtain a bipolar offset current different from 0.5 mA.
1000
800
600
400
200
µA
Figure 19. Bipolar Offset Current vs. External Resistor
500
1000
1500
EXTERNAL RESISTOR
2000
2500
3000
3500
4000
AD650
Ω