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I have two AD5241 1 Mohm digital potentiometers.
Individually they work properly. But when I try to build them into a single circuit to have two each 1 Mega ohm control points, I end up with two 500 Kohm resistors, which suggests to me that they are operating in parallel. I'm not sure why this is happening though.

Here is a picture of the setup.
W|A W|B are connected to a multi-meter. I'm trying to keep it as simple as possible for this question. Maybe Ill return to KiCAD, and label that though.

I'm programming them, via I2C, their individual RT= 1 Mohm. Unless both are connected. Which is the issue. They remain programmable, but the scale is from 500 Kohm to 0 ohm, when both are in circuit instead of 1 Mohm each.

What am I doing wrong here?

Improved picture

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  • \$\begingroup\$ The W1/B1 pins are not connected to anything. (Neither are the I²C lines.) This circuit, as shown, does not do anything. \$\endgroup\$ Commented Feb 27 at 21:09
  • \$\begingroup\$ W|A W|B are connected to a multi-meter. I'm trying to keep it as simple as possible for this question. \$\endgroup\$ Commented Feb 28 at 12:58
  • \$\begingroup\$ " end up with 2 500K ohm resistors". measured how? Where? \$\endgroup\$ Commented Feb 28 at 13:16
  • \$\begingroup\$ @DavideAndrea measured A to B or respective programmatic levels when coded A to W or B to W, using a pair of digital multi meters. \$\endgroup\$ Commented Feb 28 at 14:34
  • \$\begingroup\$ When you suspect bad wiring, then show the wiring (or the board layout). What specific multimeters are you using, and what is their input impedance? \$\endgroup\$ Commented Feb 28 at 15:34

2 Answers 2

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According to the datasheet:

Wiper position programming defaults to midscale at system power on.

So if you are powering them up and then measuring each pot wiper to ground (you have A1 grounded) or wiper to B1, 500k is what you should see for each one.

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  • \$\begingroup\$ Im programing them, via i2c, their individual RT=1Mohm. Unless both are connected. Which is the issue. They remain programable, but the scale is from 500K ohm to 0 ohm, when both are in circuit instead of 1M ohm each. \$\endgroup\$ Commented Feb 28 at 13:03
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EDIT: none of the below applies because it was based on incorrect information from OP.

to have 2 mega ohm control points

To get a total of 2 MOhm, you need to connect them in series. Your schematic diagram shows the A1 terminal connected to ground on both of them. That prevents you from connecting them in series.

I also note that you did not supply a connect a voltage to Vss. Vss has to be connected to a power supply between 0 and - 5 V.

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    \$\begingroup\$ I don't think they want one 2 Meg control point, they want two 1 Meg control points. The way it's written makes it confusing. \$\endgroup\$ Commented Feb 27 at 22:48
  • \$\begingroup\$ @ GodJihyo "the way it's written makes it confusing." It sure is! \$\endgroup\$ Commented Feb 27 at 23:51
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    \$\begingroup\$ the goal is to have 2, 1M ohm control points, not 1 2Mohm control point. indeed, I forgot to draw this, but vss is tied to gnd in circuit. I guess Ill fix my picture. \$\endgroup\$ Commented Feb 28 at 13:05

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