Hello,
I am currently evaluating the HMC305S digital attenuator and I am observing unexpected insertion loss behavior.
Test setup:
- Standalone HMC305S measured with a VNA
- No control interface activity during the initial measurement
Observed behavior:
- Device unpowered (VCC = 0 V): insertion loss ≈ 15.5 dB (signal passes through the device)
- Device powered (VCC applied, no control activity): insertion loss ≈ 21.5 dB
- Device powered with SPI control: attenuation range ≈ 6–21.5 dB
It appears that the entire attenuation range is shifted by approximately +6 dB compared to the expected 0–15.5 dB range specified in the datasheet.
My questions:
1. What is the expected default attenuation state at power-up if the control pins are left floating?
2. Do the control inputs have internal pull-up or pull-down resistors that could result in a non-zero default attenuation state?
3. Are there specific pin biasing or power-up sequencing requirements needed to achieve the datasheet-specified minimum insertion loss?
4. Is a +6 dB offset consistent with normal device operation?
I can provide additional details (frequency, supply measurements, PCB layout, etc.) if needed.
Thank you in advance for your support.
