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ams simulator vs spectre: empty schematic view

rosinc

Hi,

Cadence version: 17.1.0.270.isr5 64bit

I have the following circuit: a resistor (100 kOhm) connected between vdd (5 V) and a current source to ground (10 uA), and a copy of this with an extra block connected to the output node of the resistor (in the image schem1.png attached). This extra block is called 'empty_schem' and it only has a pin (output direction) with nothing connected (schem2.png). I would expect the output node of the resistor to be at 4 V but when I run a dc simulation with the ams simulator the output node where the 'empty_schem' block is connected has a different voltage (3.9604 V, also visible in the image) with respect to the same node without the block (exactly 4 V). Whereas, if I run the simulation with spectre as simulator, the two nodes have the same voltage (4 V as expected). What is the explanation of this? I could not find a solution to this problem. I need to understand this because I, sometimes, use these empty blocks to speed up the simulation: if I have big arrays of instances, I set some of them to be empty in the config.

Best,

Rosario

Parents

  • Hi Rosario,

    This is because an interface element (an instance of a connect module) gets inserted between the out pin of empty_schem and the net out2. Because there's nothing electrical inside the empty schematic, it assumes that it is logic, and inserts a logic to electrical connect module. This has some output impedance, and that is interfering with operating point as a result.

    You can see this by using Launch->Plugins->Mixed Signal Opts->AMS and then AMS->Display Partition->Initialize (after you've run the simulation), and then do AMS->Display Partition->IE Information. You'll see details of the interface element on the out2 net.

    You can avoid it by (inside empty_schem) drawing a wire from the out pin, and then selecting that wire and doing edit properties. Add a property (of type string) with the property name netDiscipline with value electrical. That will prevent the insertion of the interface element and  you'll then see the node voltage is 4 on out2.

    Regards,

    Andrew.

Reply

  • Hi Rosario,

    This is because an interface element (an instance of a connect module) gets inserted between the out pin of empty_schem and the net out2. Because there's nothing electrical inside the empty schematic, it assumes that it is logic, and inserts a logic to electrical connect module. This has some output impedance, and that is interfering with operating point as a result.

    You can see this by using Launch->Plugins->Mixed Signal Opts->AMS and then AMS->Display Partition->Initialize (after you've run the simulation), and then do AMS->Display Partition->IE Information. You'll see details of the interface element on the out2 net.

    You can avoid it by (inside empty_schem) drawing a wire from the out pin, and then selecting that wire and doing edit properties. Add a property (of type string) with the property name netDiscipline with value electrical. That will prevent the insertion of the interface element and  you'll then see the node voltage is 4 on out2.

    Regards,

    Andrew.

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