Analog Input Stage¶

The signals acquired by the OPX using the analog input channels are first passed through an analog input stage and then digitized. Here we discuss the voltage limits, gain and offset calibrations that can be applied to the signal to be digitized.

Acquiring analog signals with the OPX¶

The signal chain can be seen in Fig. 1. The signal source can be either AC or DC coupled.

Note

The OPX input has a common-mode voltage of 0.25 V when measured with 50 ohm.

DC coupled signal source

If the signal source is DC coupled and has 50 ohm DC resistance to ground, it can be directly connected to the analog input. This can be seen in Option 1 of Fig. 1
AC coupled signal source

If the signal source is AC coupled and does not have a 50 ohm resistance to ground, a bias T is needed in order to bias the input amplifier. The required setup can be seen in option 2 of Fig. 1. In this case, the source will still need to have a 50 ohm AC impedance throughout its bandwidth for proper operation.

Properties of the analog input stage¶

The analog input has the following properties:

Input impedance: 50 ohms
ADC resolution: 12 bits
Input bandwidth: 400 MHz
Full voltage range: 1.0 Vpp into 50 ohm or 4 dBm (nominal)
Variable gain: -12 dB to 20 dB in steps of 1 dB

Setting the voltage gain¶

The voltage gain can be set by adding a "gain_db" field to the "analog_inputs" field of the controller in the config. Here we show an example with the relevant fields:

config = {
    'version': 1,
    'controllers': {
        'con1': {
            'analog_inputs': {
                1: {'offset': 0.0, 'gain_db': -1},
                2: {'offset': 0.0, 'gain_db': 1},
            }
        }
    },
}

Note

When changing the gain, the input DC offset needs to be recalibrated.