I have a AD8000 SOIC package in noninverting configuration (Fig. 50 datasheet). Rf=439 ohms, Rs=0ohms, RL is open circuit, Rg = open circuit, 0.1 uF decoupling caps at supply pins. I am using +/-5V supply. I am interested in the performance at 1.25 GHz. I use -20 dBm input and get -26 dBm output at 1.25 GHz. I don't think this matches the performance shown in Fig.5 of datasheet. Can you suggest what I could change to make it work as expected?
I added 10 uF decoupling caps. at the V+ and V- terminals and also made the feedback close to 431 ohms. Now I get a 3 dB loss at 1.25 GHz (small signal, -10 dB input), but table 5 in datasheet claims a 3 dB bandwidth of 1.58 GHz. The numbers I get match Figure 5. Why does table 5 not match figure 5? What is the difference from?
Could you run a frequency sweep on your setup? By that we could see the actual response of the amplifier. In Figure 5, at G=+1, the gain is rolling off rapidly after 1 GHz. This means that a very small change in frequency is a drastic decrease on the gain. The best way to see this is on a frequency sweep on your current setup.
The variation on your result and on the table 5 is due to parasitic capacitance present on your board. This greatly contributes on the decreased bandwidth as compared to the defined value in the said table.
Thanks for your response.
The figure 5 of the datasheet does not match Table 5 (Gain = 1 for SOIC). Why is that?
Could you elaborate on the parasitic capacitance you refer to. I have made sure there is no ground plane underneath the high speed signal traces. I do have a ground plane underneath the chip as in the datasheet (smaller than the datasheet with fewer vias). Let me know if you have layout recommendations. I have tried to follow those in your datasheet.
Also, if would be great if you have a sample gerber for this AD8000 SOIC chip, noninverting circuit. (Figure 50)
Thanks and look forward to your response.
Figure 5 does match the data in Table 5. Please note that the graph is in log scale.
These discussions would help in your layout considerations. AD8000&AD8001 PCB design issues and Same function by using same pcb design for AD8001&AD8000.