AD650SD【PDF 15/20 页】IC V-F/F-V CONV 1MHZ 14-CDIP

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Sheet目录38

11页 12页 13页 14页 [15页]16页 17页 18页 19页

Data Sheet

AD650

Rev. E | Page 15 of 20

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 k?is connected from Pin 13 to

Pin 14 and the wiper is connected to the positive supply

through a 250 k?resistor. A potential of about 0.6 V is

established across the 250 k?resistor, and the 3 糀 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 smalltypically less than 3 糀. This

technique is shown in the Applications section of this data

sheet; the autozero circuit uses this technique.

The bipolar offset current is activated by connecting a 1.24 k?

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 ?0%. 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 k?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

to obtain a bipolar offset current different from 0.5 mA.

Figure 19 shows the relationship between the bipolar offset

current and the value of the resistor used to activate the source.

礎

200

500

4000

EXTERNAL RESISTOR

400

600

800

1000

1500

2000

2500

3000

3500

Figure 19. Bipolar Offset Current vs. External Resistor

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