What is the drive capability and output impedance of this output? It doesn't seem to be specified in the datasheet of either this or similar parts.
The drive capability is about 2mA and the output impedance is approximately 5 ohms. The circuit has a high gm (high open loop gain) but has feedback (closed loop gain = 1).
Thanks a lot,
I have a question regarding VCM.
Registry map mentions that You can turn on VCM Servo by setting Bit 0 in Register 0x0F to logic high.
Can the SPI Controller program given by AD do that? We cant find that option in there...
Will this raise the level of VCM above 0.11V, which we measure right now when this bit is in logic low?
Is there be a difference between when VCM Servo is on, and to simply externally adding VCM as offset voltage in the low frequency signal?
Main problem we have with AD9268 Evaluation board is that we want to convert input frequencies from DC to 5MHz, and when we try to do that, signal we get is attenuated - most probably due to coupling capacitors at the inputs.
To bypass them we provide sinusoidal signal behind C406/C407 capacitors - in single ended form (only behind C406). When we dont add significant offset voltage our whole signal looks chopped (like quatized with 4bit ADC).
When Offset voltage of ca. 0.9V is added to the signal, we get quite smooth looking signal even for f<100Hz, but after zooming in (software used is Visual Analog from AD Page), we see cyclical loss of signal: sinus is building up, then there is a drop (but not to zero), and continued building up - our interpretation of this is like some of the least significant bits are dropped.
We also tried to use ADA4938-2 as dedicated way to drive the ADC with low frequency signals. Whenever we use differential signals at inputs of the ADC our signal is scrambled. We try supplying the signal through SMA input for channel A (single ended) and fully differential, when soldered behind C406/C407. VCM for ADA4938-2 was provided externally - 0.9V DC, as we cannot turn on VCM Servo using SPI Controller.
What is the proper way to configure this ADC to work with DC-5MHz range?
II am trying to decipher all the conditions you are testing. It sounds like you have tried multiple input configuration but it is hard to tell exactly what you were seeing under each condition. Let's try to focus on just one condition at a time.
On the evaluation board the VCM servo is not typically required. It is meant for use only when a large resistance is placed between the VCM output and the analog inputs. The VCM for the AD9268 should be 0.9V for proper operation of the ADC. If you are reading 0.11V then you have something connected improperly on the inputs.
It sounds like in at least one case you are trying to drive an input form a signal source after the series AC coupling caps (C406/407). If this is the case then you are providing a DC path to ground through your signal source unless your signal source can provide a DC offset of 0.9V. The AC coupling caps will indeed attenuate very low frequency signals as you are seeing.
To use the ADA4938-2 (assuming you are connecting its evaluation board up to the AD9268 evaluation board) you'll need to remove the AC coupling caps form the AD9268 evaluation board and set the proper load impedance for the ADA4938 with the AD9268 input network and set the Vocm pin of the ADA4938-2 to 0.9V to be compatible with the ADC (I believe the ADA4938-2 should support this, but it is probably worth checking with the amplifier applications team to be sure).
You should not need to use the VCM Servo in either of these cases and I would recommend not focusing on that.
The main idea here is that the ADC inputs need to have a common mode of 0.9V or you will not see proper operation. You would likely seem some form of clipping of the signal because you will not have the full voltage range available on the ADC inputs. In order to have a DC-coupled input to the ADC you need to remove the series AC coupling capacitors and meet 1 of the following conditions:
If you are still running into issues are trying the suggestions above let me know and please provide some data such as FFT plots and perhaps a photo or drawing of your configuration so that I can help you troubleshoot more efficiently.
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