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Driving LT5571 with a single ended DAC

I'm trying to drive the LT5571 with a single ended DAC (MCP4912) for modulation of a narrow band signal (~10kHz max). 

Because of the nature of the low frequency narrow band signal, I'm not overly concerned with degradation of modulator linearity and/or the advantages of using a differential DAC.

However, I'm not entirely sure what the best way to drive the two differential (I and Q) inputs of the LT5571 are, especially considering the differential input requires a common mode voltage of around 0.5V.

I feel like the most simple way would be to bias one of the differential pins at 0.5V using a resistive divider, and then simply connect the DAC output to the other differential input with a 50 Ohm load to ground (We don't have any need to optimize for the full swing of the DAC).

Assuming this works, my other concern is the slight difference in common mode voltages across boards due to resistor tolerances (The goal is to use multiple modulators to create a phased array) which may affect performance of the array.

Is this method suitable? If not, what would be the recommended way to drive the differential inputs (keeping in mind for this specific application we value the simplicity of circuit/layout much more highly than performance increases).

Thanks in advance.

  • Your proposed design would work fine. Keep in mind that you will have 6dB lower gain and degradation in linearity. High precision resistor should be used. You will likely need to adjust your DAC output current to generate precise matched 0.5V DC common-mode voltage. Poor DC common-mode voltage matching will result in large carrier leakage.