If we use HMC900LP5 and HMC960LP4 as single-end input, how should we handle input pins?
Connect to GND? or Open?
Best Regard, Tomo
Additional comments for this configuration:
1. It is assumed that the HMC960 output is DC and differentially coupled to the HMC900 input. If so, the CM (common mode output bias) pins of the HMC960 should be biased to 2.5V to match the common mode input voltage of the HMC900;
2. The single-ended output of the HMC900 cannot drive a DC coupled load connected to ground, only AC coupled single-ended loads are allowed. See datasheet page 3-18 Output Driver description:
3. HMC900 CM (common mode output bias) pins should be biased to 2.5V typically, but can be between 1.5V & 3.5V (see datasheet);
Assuming AC coupling, then both inputs should be DC biased (to meet the common mode input voltage specification in the datasheet) and the undriven input pin should have a capacitor to ground.
If DC coupled, then the undriven input should be DC biased to the same DC level (as the driven input pin) and a capacitor connected from the unused input pin to ground.
Thank you for your answer.
How much value of capacitor you recommended?
The capacitor and the effective input resistance will create a low frequency cut-off. So, the capacitor value needs to be set to ensure the cut-off is sufficiently low for the spectrum of interest.
In the AC coupling case, the effective input resistance (as seen from the source) will be the parallel combination of the internal input resistance of the HMC900 (1kOhm differential) and the DC common mode bias arrangement. The reactance of the capacitor at the minimum expected input signal frequency must be much less than (<1/10) the effective input resistance.
In the DC coupling case, the input resistance is 1kOhm so the reactance of the capacitor at the minimum expected input signal frequency must be <<1kOhm.
Using condiiton is below.
- Single end I in, Single end Q in ==> HMC960LP4E ==> HMC900LP5E ==> single end I out, single end Q out
- Frequency : 10MHz, 100MHz
Please tell me the capacitor value at 10MHz and 100MHz using.
Thank you for your support.
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