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Qorvo 1800MHz Small Cell RF Card Hardware User’s Guide
Hardware User’s Guide
5.13 ADC Input Protection
It is important that the ADC inputs of the RFSoC device on ZCU111 be protected from out-
of range external inputs.
1
The
Rx LNA’s can therefore be shut down or bypassed through
combination over-voltage detection and user controls via software.
5.13.1
RX-side ADC Over-voltage Protection
An Analog Devices AD8361 mean-responding power detector at the output of the
QPL9096 (LNA0) in each channel of RX signal chain monitors the signal level driving into
the ADC. The power detector is resistively coupled at the output of QPL9096 LNA through
453
Ω to form a voltage divider with the input impedance of AD8361, resulting in an
attenuation factor of ~0.14. The protection circuit is nominally set to trigger when the LNA
output r5 dBm (50Ω), to account for estimated combined insertion loss of 4 dB
through TQQ0302 BAW filter, pi network, balun and LPAM connector to the RFSoC ADC
inputs on ZCU111.
The linear voltage output of the RMS detector drives a non-inverting op amp with make-
up gain of 4 to compensate for the attenuation of the resistive coupling, followed by an
open-loop op-amp comparator whose voltage threshold is set to trigger at 1.2 V, which
corresponds to the maximum input level of 1V pk-pk at the RFSoC ADC input pins. Under
this condition the LNA immediately switches to the (LNA OFF, Bypass OFF) state,
whereby the RFSoC ADC input will reduce to signal ground through a 10K pull-down
resistor at the output of the LNA0. This mechanism is intended to protect the RFSoC ADC
inputs from inadvertent over-voltage, and is independent of any operational state of the
ZCU111.
Note that the LNA over-voltage detection signals, CH1_RX_OV and CH2_RX_OV, are
routed back to the ZCU111 GPIO so that software can reliably detect this hardware
condition.
Figure 19- Rx-Side ADC over-voltage protection
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The RF-ADC input buffer contains an internal over voltage feature offering protection for signals in the range defined
in the Zynq Ult RFSoC Data Sheet: DC and AC Switching Characteristics (
). Full-scale input voltage
is rated at 1V pk-pk (1 dBm into 100 Ω on-die termination). Signals exceeding this maximum are not permitted, and
care must be taken externally to ensure that such voltages are not presented to the RF-ADC inputs.
