I want to use the AD8310 with a DC-coupled input. I am using the setup from the datasheet in Figure 38 (page 19) reproduced here:
However, I have an extremely poor dynamic range. The dynamic range of this circuit is advertised through Figure 39 (page 19) of the datasheet, which is reproduced below:
I am not getting this dynamic range no matter what I adjust the potentiometer to. The lower end of the range is limited at an input of about -30 dBm.
Any ideas how I could troubleshoot this?
If it helps, if I use the basic AC-coupled configuration (show below), I DO indeed achieve the advertised dynamic range.
Thank you for your time!
Also another question. Typically, if one wants to use the AD8310 for ac-coupled low frequency operation, the following setup is used:
If I wanted to use a DC-coupled input AND operate in lower frequencies, how would I modify the BFIN and OFLT pins?
When dc coupled, the AD8310 detects DC offsets as valid inputs. To check quickly if DC offset is limiting the dyynamic range of the 8310, just measure the dc voltage across the inputs of the 8310 with your signal source connected bu turn to a low level (<-70 dBm if possible). The RF level of -30 dBm correspond to 7mVrms, so if that level of DC occurs across the inputs, it will become the tangential sensitivity level. DC offsets can arise from the signal source itself, or if the source resistance is significantly different from 50 ohms in the circuit using the AD8138.
Thank you for your suggestion Benjamin, I will give it a try.
For my other question.... To operate in lower frequencies, the AD8310 OFLT pin is tied to ground through a 1 uF capacitor. If I wanted a DC-coupled input AND operate in lower frequencies, do I tie a 1 uF capacitor to the 1.9V resistor divider instead of ground?
When DC coupling the AD8310, as seen in figure 38, you turn off the input offset correct loop. This is what the resistor divider to OFLT pin does. So since the input offset correction loop is off, you do not need to put a 1uF cap on the OFLT pin. The 50 ohm potentiometer in figure 38 is now trimming the DC offsets of both the AD8138 and the AD8310.
Hope this helps,
Thank you for your reply, I think I'm starting to get a clear picture. So for a DC-coupled input and low-frequency applications, is it recommended to tie the BFIN pin to ground with the 2.7 nF capacitor? Or is that for AC-coupled applications only?