Help understanding schematic from datasheet

Using Arduino General Electronics
1

Hi!

I want to control the frequency of the function generator IC XR-2206 with voltage (instead of the usual way with a pot).
Figure 10 (see attachment) in the datasheet show a way (I think): http://www.jaycar.com.au/images_uploaded/XR2206V1.PDF

I do not fully understand how it works. I plan to use a DAC that output 0-3V as sweep input.

  1. In the middle of the figure it says +-3V.
    Pin 7 outputs 3V and the sweep input is 3V.
    When sweep input is 0V Pin 7 is +3V, correct?
    What comes the negative voltage from? Is it because the sweep input is 3V?

  2. What values for Rc and R?
    The datasheet reads 1uA to 3mA (max) on pin 7. How do I calculate the current on pin 7 in this case?

  3. I think the GND of the Arduino and DAC should be connected to GND of XR-2206? Is this correct?

/Olof

sweepInput.jpg

2
  1. I think the GND of the Arduino and DAC should be connected to GND of XR-2206? Is this correct?

Yes.

  1. What values for Rc and R?

What ever value they need to be to cut down the sweep input to 3V maximum. If the sweep input is 3V then Rc = 0 and R = infinity.

The datasheet reads 1uA to 3mA (max) on pin 7.

Can't see this can you say what page it is on.

  1. In the middle of the figure

What is the figure number for this?

3

The pins 7 and 8 are current-controlled, and internally held at 3V above pin 12.

You are allowed to sink upto 3mA from them, this current controls the frequency
(in conjunction with the timing capacitor).

In figure 10, you can remove R and just use Rc (make sure it is at least 1k to limit current
to 3mA. Only provide an input voltage of 3V or below (no need to go negative, but
you can if you want, but the value of Rc will need to be high enough to keep
within the 3mA limit.

There is a section in the text of the datasheet about this.

What's actually happening in the chip is that there is an internal op-amp whose
inverting-input is connected to pins 7 and 8, while its non-inverting input is biased
to 3V above pin 12. The timing capacitor will be charged and discharged using
this current somehow by a feedback netwoek, producing a triangle wave.

4

Thanks Mike and MarkT!

I misread + 3V - on figure 10. Thought it read +-3V but of course it is just 3V.

Thanks again for taking your time to help me.