Unusually High Output Offset Voltage in AD8337 LTSPICE Model

Question

I'm planning on using the AD8337 VGA for a new design. When simulating the part, however, I noticed unexpectedly high output offset voltage present, seeming to disagree with quoted datasheet specifications. The output offset voltage seems to be 50mV higher than expected, but the characteristic shape of the offset across gain setting seems to match. 
 I'm using Revision D of the SPICE model released in 2019 (the model currently posted on the website). I've attached screenshots of my LTSPICE circuit and screenshots from the datasheet I'm referencing (pg. 22 of AD8337 datasheet). Notice the difference in output offset voltage between the simulation and the datasheet screenshot.

It seems like the SPICE model for this IC has historically had a lot of issues (though these seem to typically involve single-supply operation), see: 
 - AD8337 / SPICE MODEL Dynamic Range 
 - Simulation not working for certain values on the Vgain pin 
 - How can I correctly connect the AD8337 spice model? 
 - AD8337 supply 
 Please ensure the SPICE actually represents true device performance. Incorrect SPICE models make me hesitate to buy ADI products.

ADIApproved · Accepted Answer

Hello govish87 Apologies for the inconvenience. As I mentioned on my last reply, the offset voltage of AD8337's SPICE Web model at +/-5V Supplies were not modeled. I'll report this internally for it to get fixed. But rest assured that in real life, the typical offset voltage performance of the part at +/-5V should look like the one on Figure 15. Regards, Paul)

ADIApproved · Answer

Hello govish87 , Currently, we are on a holiday break and we kindly request for your patience and understanding. We'll verify the offset voltage issue on dual supplies once we get back at the office. Apologies for the inconvenience caused. I also agree that the spice model has an issue with single supply operation as I've already reported this internally in the past. Best regards, Paul

ADIApproved · Answer

Hello govish87 , I've also used the SPICE web model of AD8337 and was able to get a result quite similar to Figure 74. (Vout = 4mVpp) Please take note that on Figure 74, Vs = +/-2.5V was used. In addition, the ramp signal applied on the Vgain pin has a frequency of 1kHz. This could be the reason why you have obtained a different result from what is shown on Figure 74. I hope this helps. Best regards, Paul

ADIApproved · Answer

Hello govish87 , - Yes, you are correct. Increasing the supply voltage will also increase the offset voltage. - As checked also, it looks like the offset voltage of AD8337's SPICE model at +/-5V Supplies were not modeled. Apologies for the inconvenience. In real life, the typical offset voltage performance of the part at +/-5V should look like the one on Figure 15. Regardless, I associate Output Offset Voltage to be a DC characteristic in any case--why is it fair to measure output offset with an AC waveform? Is there a particular part of the SPICE model that may be responsible for this discrepancy between DC and AC offset performance, and is that representative of true device behavior? - Please correct me if I'm wrong. Are you referring to the offset voltage plots of Figure 15 and Figure 74? Please take note that the voltage at the VGAIN pin continuously changes, which effectively changes the gain and thus, the output offset voltage. The offset voltage specification from the electrical characteristics table were only taken at a specific gain setting (VGAIN = 0.7V, 24dB). The output offset voltage spec is a DC spec. Ex: Looking at the table below, the acceptable output offset voltage value for linear operation is from -25mV to +25mV, (+/-5mV, typically) when Vs = +/-2.5V | Now, if we take a look again at Figure 74, at around Vgain = 0.7V, we can get an offset voltage value of approximately -1.5mV. This value directly correlates with the Output Offset Voltage spec from the table above. -1.5mV is within the accepted output offset voltage value specified. Best regards, Paul

SLA Status	Assignee	Support Status
SLA Closed. Met after 54 hours	maaarmas MStokowski	Awaiting Company Response

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Unusually High Output Offset Voltage in AD8337 LTSPICE Model

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