As shown above, when DAC = 0V, input 0.8v 2.5MHZ signal, the output is 0.35v. DAC = 0.6V, input 0.08v 2.5MHZ signal, the output is only 0.8v, rather than zoom 40db. why?
Apologies for the late response.
AD8338's Linear-in-dB Gain goes by this equation:
So for your DAC range of 0.1 V to 0.6 V, the Gain in dB should have a range of 0 dB to 40 dB differentially. Effectively, the single-ended gain would be -6dB to 34 dB or 0.5V/V to 50V/V. With your input as 80 mV, that would make your output swing at 40 mV to 4.0 V, which is beyond the operating point of the VGA. The part will have an optimum operation on these ranges:
Yes, the gain should be 40dB. However, with an input of 80 mV at a gain of 40 dB(100V/V) the output would be 8.0 V, which is obviously beyond the output swing limitation of AD8338. Please see page 3 of the datasheet for details on the operating range of this part. Reduce the value of your gain control (DAC) to a linear operation of AD8338 gain.
In theory, when vin = 0.08v, 40db should be saturated. But the DAC is linearly adjusted from 0.1v to 0.6v, the output has not been saturated. At DAC = 0.6V, the output is only 0.8v
How did you measure the output? Differential or Single-ended?
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