Fast Analog Inputs

This page contains detailed specifications and performance measurements for Original Generation fast analog inputs.

The STEMlab 125-14 board analog front-end features 2 fast analog inputs.

General Specifications

Number of channels	2
Sample rate	125 Msps
ADC resolution	14 bits
Input coupling	DC
Absolute maximum input voltage rating	LV: 6 V (1500 V ESD) HV: 30 V (1500 V ESD)
Overload protection	Protection diodes (under the input voltage rating conditions)
Connector type	SMA
Input stage voltage ranges [1]	LV (±1 V) HV (±20 V)
Bandwidth	50 MHz (-3 dB)

[1]

Measurement performance is specified within this range.

Note

Overload protection applies to low frequency signals. For input signals containing frequency components above 1 kHz, where capacitor divider comes into play, the full scale value defines the maximum permissible input voltage.

Note

The SMA connectors on the cables connected to Red Pitaya must comply with the MILC39012 standard. The centre pin must be of suitable length, otherwise the SMA connector installed in Red Pitaya will mechanically damage the SMA connector. The centre pin of the SMA connector on Red Pitaya will lose contact with the board and the board will not be repairable due to the mechanical damage (separation of the pad from the board).

Input stage schematics

Figure 4.39 Fast analog inputs schematics

Input coupling

Fast analog inputs are DC coupled. Input impedance is given in the picture below.

Figure 4.40 The input impedance of fast analog inputs

Input bandwidth

Bandwidth

50 MHz (-3 dB)

In the picture below, the Frequency Response - Bandwidth of fast analog inputs is shown. Measurements are taken using an Agilent 33250A signal generator as a reference. The measured signal is acquired using remote control commands. An amplitude voltage is extracted from the acquired signal and compared to the reference signal amplitude.

Figure 4.41 The bandwidth of fast analog inputs

Because of the maximum sampling rate of 125 MS/s when measuring signals above 10 MHz, we have used sin(x)/x interpolation to get more accurate results of Vpp voltage and, with that, more accurate measurements of analog bandwidth. When measuring signals above 10 MHz, similar results should be obtained without interpolation or directly with an Oscilloscope application and P2P measurements.

Notice: When making measurements without interpolation, you need to extract the maximum and minimum of the acquired signal using a complete 16k buffer. When using P2P measurements on an oscilloscope, you need to take the maximum value shown as a measurement result. An example of sin(x)/x interpolation for a 40 MHz signal is shown in the picture below (right).

Note

In the picture, only 10 samples of 16k buffer are shown to represent a few periods of 40 MHz signal.

Figure 4.42 Sin(x)/x Interpolation

Input noise

Measurements refer to a high gain (LV ±1 V) jumper setting, with limited environmental noise, inputs and outputs terminated, output signals disabled, and the PCB grounded through SMA ground. Measurements are performed on 16k continuous samples at full rate (125 MS/s). (Typical full bandwidth std(Vn) < 0.5 mV). The noise spectrum shown in the picture below (right) is calculated using FFT analysis on N = 16384 samples sampled at Fs = 125E6 MS/s.

Figure 4.43 Noise distribution

Figure 4.44 Noise level

Input crosstalk

Crosstalk measurements were performed between input channels 1 and 2 for both LV and HV modes.

	Up to 30 MHz		Above 30 MHz
IN1 IN2	LV	HV	LV	HV
LV	>55 dB	>60 dB	40 dB	40 dB
HV	>40 dB	>50 dB	35 dB	35 dB
IN2 IN1	LV	HV	LV	HV
LV	>55 dB	50 dB	40 dB	40 dB
HV	>55 dB	>50 dB	>40 dB	>35 dB

Typical performance:

• 65 dB @ 10 kHz

• 50 dB @ 100 kHz

• 55 dB @ 1 M

• 55 dB @ 10 MHz

• 52 dB @ 20 MHz

• 48 dB @ 30 MHz

• 44 dB @ 40 MHz

• 40 dB @ 50 MHz

The typical performance characteristics were measured on LV jumper setting on both channels. The SMA connectors not involved in the measurement are terminated.

Input harmonics

Amplitude	Harmonics
-3 dBFS	-45 dBc
-20 dBFS	-60 dBc

The typical measurements refer to the LV jumper setting, inputs matched and outputs terminated, outputs signal disabled, and PCB grounded through SMA ground.

Input SFDR

Input SFDR

< -90 dBFS

Measurements refer to the LV jumper setting, inputs, and outputs terminated, outputs signal disabled, and the PCB grounded through SMA ground. In the pictures below, typical performances of Red Pitaya fast analog inputs are shown. For the reference signal generation, we have used the Agilent 33250A Signal generator. For the reference spectrum measurements of the generated signal, we have used the Agilent E4404B Spectrum analyzer. The same signal is acquired with the Red Pitaya board and FFT analysis is performed. Results are shown in the figures below, where Red Pitaya measurements are on the right.

Figure 4.45 Measurement setup

Reference signals

1. Reference signal: -20 dBm, 2 MHz

   

2. Reference signal: -20 dBm, 10 MHz

   

3. Reference signal: -20 dBm, 30 MHz

   

4. Reference signal: 0 dBm, 2 MHz

   

5. Reference signal: 0 dBm, 10 MHz

   

6. Reference signal: 0 dBm, 30 MHz

   

7. Reference signal: -3 dBFS, 2 MHz

   

8. Reference signal: -3 dBFS, 10 MHz

   

9. Reference signal: -3 dBFS, 30 MHz

   

Due to the natural distribution of the electrical characteristics of the analog inputs and outputs, their offsets and gains will differ slightly across various Red Pitaya boards and may change over time. The calibration coefficients are stored in EEPROM on the Red Pitaya and can be accessed and modified with the calibration utility:

Input DC offset error

DC offset error

< 5% FS

Input gain error

Jumper settings	Gain error
LV	<3%
HV	<10%

Further corrections can be applied through more precise gain and DC offset calibration.

Analog inputs calibration

Calibration processes can be performed using the Calibration application or using the calib command line utility. To calibrate the Red Pitaya using the Calibration application, simply select System -> Calibration and follow the instructions.

Calibration using calib utility

Start your Red Pitaya and connect to it via SSH.

root@rp-xxxxxx:~# calib
calib version 2.00-0-f6ded7198

Usage: calib [OPTION]...

OPTIONS:
 -r    Read calibration values from eeprom (to stdout).
       The -n flag has no effect. The system automatically determines the type of stored data.

 -w    Write calibration values to eeprom (from stdin).
       Possible combination of flags: -wn, -wf, -wfn, -wmn, -wfmn

 -f    Use factory address space.
 -d    Reset calibration values in eeprom from factory zone. WARNING: Saves automatic to a new format

 -i    Reset calibration values in eeprom by default
       Possible combination of flags: -in , -inf.

 -o    Converts the calibration from the user zone to the old calibration format. For ecosystem version 0.98

 -v    Produce verbose output.
 -h    Print this info.
 -x    Print in hex.
 -u    Print stored calibration in unified format.

 -m    Modify specific parameter in universal calibration
 -n    Flag for working with the new calibration storage format.

The EEPROM is a non-volatile memory, so the calibration coefficients will not change during Red Pitaya power cycles, software upgrades via Bazaar, or manual changes to the contents of the SD card. An example of reading calibration parameters from the EEPROM with verbose output:

root@rp-xxxxxx:~# calib -r -v
dataStructureId = 5
wpCheck = 53
count = 28
DAC Ch1 Gain (1) = 2674690              # OUT1 gain coefficient
DAC Ch1 Offset (2) = -69                # OUT1 DC offset
DAC Ch2 Gain (3) = 2692407              # OUT2 gain coefficient
DAC Ch2 Offset (4) = -94                # OUT2 DC offset
ADC Ch1 Gain 1/1 (9) = 2817122          # IN1 gain coefficient for LV (±1V range)  jumper configuration
ADC Ch1 Offset 1/1 (10) = -159          # IN1 DC offset for LV (±1V range)  jumper configuration
ADC Ch2 Gain 1/1 (11) = 2811646         # IN2 gain coefficient for LV (±1V range)  jumper configuration
ADC Ch2 Offset 1/1 (12) = -126          # IN2 DC offset for LV (±1V range)  jumper configuration
ADC Ch1 Gain 1/20 (17) = 3113286        # IN1 gain coefficient for HV (±20V range) jumper configuration
ADC Ch1 Offset 1/20 (18) = -186         # IN1 DC offset for HV (±20V range) jumper configuration
ADC Ch2 Gain 1/20 (19) = 3115407        # IN2 gain coefficient for HV (±20V range) jumper configuration
ADC Ch2 Offset 1/20 (20) = -148         # IN2 DC offset for HV (±20V range) jumper configuration
ADC Ch1 AA 1/1 (33) = 32147             # IN1 FPGA filter coefficient AA for LV
ADC Ch1 BB 1/1 (34) = 276423            # IN1 FPGA filter coefficient BB for LV
ADC Ch1 PP 1/1 (35) = 9830              # IN1 FPGA filter coefficient PP for LV
ADC Ch1 KK 1/1 (36) = 14260634          # IN1 FPGA filter coefficient KK for LV
ADC Ch2 AA 1/1 (37) = 32147             # IN2 FPGA filter coefficient AA for LV
ADC Ch2 BB 1/1 (38) = 276423            # IN2 FPGA filter coefficient BB for LV
ADC Ch2 PP 1/1 (39) = 9830              # IN2 FPGA filter coefficient PP for LV
ADC Ch2 KK 1/1 (40) = 14260634          # IN2 FPGA filter coefficient KK for LV
ADC Ch1 AA 1/20 (49) = 16901            # IN1 FPGA filter coefficient AA for HV
ADC Ch1 BB 1/20 (50) = 193419           # IN1 FPGA filter coefficient BB for HV
ADC Ch1 PP 1/20 (51) = 9830             # IN1 FPGA filter coefficient PP for HV
ADC Ch1 KK 1/20 (52) = 14260634         # IN1 FPGA filter coefficient KK for HV
ADC Ch2 AA 1/20 (53) = 16901            # IN2 FPGA filter coefficient AA for HV
ADC Ch2 BB 1/20 (54) = 193419           # IN2 FPGA filter coefficient BB for HV
ADC Ch2 PP 1/20 (55) = 9830             # IN2 FPGA filter coefficient PP for HV
ADC Ch2 KK 1/20 (56) = 14260634         # IN2 FPGA filter coefficient KK for HV

An example of reading the same calibration parameters from EEPROM with non-verbose output, suitable for editing within scripts:

root@rp-xxxxxx:~# calib -r
                    1             2674690                   2                 -69                   3             2692407
                    4                 -94                   9             2817122                  10                -159
                   11             2811646                  12                -126                  17             3113286
                   18                -186                  19             3115407                  20                -148
                   33               32147                  34              276423                  35                9830
                   36            14260634                  37               32147                  38              276423
                   39                9830                  40            14260634                  49               16901
                   50              193419                  51                9830                  52            14260634
                   53               16901                  54              193419                  55                9830
                   56            14260634

You can write the changed calibration parameters using the calib -w command:

1. In the command line (terminal), type calib -w.

2. Press enter.

3. Paste or write new calibration parameters.

4. Press enter.

root@rp-xxxxxx:~# calib -wn
                    1             2674690                   2                 -69                   3             2692407
                    4                 -94                   9             2817122                  10                -159
                   11             2811646                  12                -126                  17             3113286
                   18                -186                  19             3115407                  20                -148
                   33               32147                  34              276423                  35                9830
                   36            14260634                  37               32147                  38              276423
                   39                9830                  40            14260634                  49               16901
                   50              193419                  51                9830                  52            14260634
                   53               16901                  54              193419                  55                9830

Should you bring the calibration vector to an undesired state, you can always reset it to factory defaults using the following command:

redpitaya> calib -d

The DC offset calibration parameter can be obtained as the average of the signal acquired with the input grounded.

The calibration parameters can be changed with the Calibration Tool. Alternatively, a reference voltage source and the oscilloscope application can be used to calculate the gain parameter. Start the oscilloscope application, connect the reference voltage to the desired input and take measurements. Change the gain calibration parameters as described above, reload the oscilloscope application and take measurements again with the new calibration parameters. The gain parameters can be optimised by repeating the calibration and measurement steps.

The table below shows typical results after calibration.

Parameter	Jumper settings	Value
DC GAIN ACCURACY @ 122 kS/s	LV	0.2%
DC OFFSET @ 122 kS/s	LV	±0.5 mV
DC GAIN ACCURACY @ 122 kS/s	HV	0.5%
DC OFFSET @ 122 kS/s	HV	±5 mV

AC gain accuracy can be extracted from Frequency response - Bandwidth.