I'm working on a design consist of LMH6554 (ADC Driver) with ADC08D1000 (Ultra HS ADC). However, I realized that my signal is unable to maintain at true zero due to the offset from the feedback loop (Possible due to the VCM from ADC). Besides that, the signal is attenuated by half due to the RO resistor (50 ohms) in parrallel with the 100 ohms resistor within the ADC.
Can I know if it will be safe to remove the 50 ohms resistor without affecting the performance of the ADC?
Also, if I removed the VCM and let the ADC operated in AC-Coupled mode, will my performance of ADC be improved?
If you don't "need" to process the DC component of your signal (as you are AC coupling the input), you'll get the best performance if you configure the ADC for AC couple mode. The LMH6554 VCMO pin should be at ground (with your +/-2.5V supplies).
I noticed that your TINA schematic has an imbalance in that the un-driven input is DC coupled while the driven input is AC coupled. It is best to keep the balance, similar to Figure 6 shown here (AC couple both inputs). May be that's why you had introduced the 0.4V biasing on the un-driven input which you may not need?
The 50ohm output resistors are there to do matching to the 100ohm differential input of the ADC and to provide LMH6554 output isolation for parasitics to the right of them. If you don't need matching (i.e. short traces and low enough frequencies compared to trace lengths), you may be able to lower the 50ohm resistors but I would keep some isolation resistance in place to keep the driver frequency response flatness. Alternatively, for larger ADC swing, you may be able to raise your gain by increasing the RF/ RG ratio (mostly by lower RG). Also, mind the input impedance in your application if you are sensitive to that?
Hope this helps.
I have a more general question about ADC drivers...
In our application there is a need to drive a dual ADC (as it is used to digitize I and Q of a QAM signal).
In this case I have to use two drivers. The question is whether there is a way to estimate the I-Q imbalance presented by differences between the two amps (I mean device production imperfection, not board related issues)?
If the matching is not specified in the datasheet of the respective part, I'd say it is very difficult to try and estimate it.
TI has dual fully differential amplifiers and some of these show the gain / phase matching between the channels. Here are some:
Device Gain Matching (dB) Phase Matching (deg.) Frequency (MHz)
LMH6517 +/-0.05 0.1 150
LMH6521 +/-0.04 0.45 200
THS4522 N/A N/A N/A
The other option is to consider:
The best way to get good matching between LMH6554 devices is to use precision resistors. Using 1% resistors will not result in good matching. The Gain, CMRR and HD2 are dominated by the resistors selected for Rf and Rg.
When building test fixtures for this device we used 0.1% resistors. If you use devices from adjacent positions on the tape you can often get better than specified accuracy.
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