AD8370 Dual Supply - Serial Interface Levels

Question

I'm using the AD8370 in a design. I'm using it as shown in figure 51 of the data sheet, DC coupled with +/- 2.5V supplies. I can't get an output from the VGA even though the power, serial control signals and input look good. The only thing I'm wondering about is the levels on the serial control signals. My control signals toggle between 0V and +2.5V. I'm wondering if they should toggle between -2.5V for a low and +2.5V for a high. Any ideas? 
 Thanks,

ADIApproved · Accepted Answer

Hi, MR_Bill, 
 As you have suspected, the AD8370's logic input HIGH is 1.8V above the Negative supply, which is -2.5V in your case. Therefor even 0V is taken as logic HIGH, and all your toggling will be ignored. The logic levels need to be translated down to -2.5V to -1.7V for logic LOW, and -0.7V to 2.5V for logic HIGH, when supplied from +/- 2.5V. Be careful that Pins 2, 7, 10, 15 and the Exposed Paddle should all be connected to -2.5V in this case. 
 Benjamin)

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AD8370 Dual Supply - Serial Interface Levels