Feedback Controllers - Making Hardware with Firmware. Part 6. Self-Calibration Related.

Steve MaslenDecember 3, 20173 comments

This article will consider the engineering of a self-calibration & self-test capability to enable the project hardware to be configured and its basic performance evaluated and verified, ready for the development of the low-latency controller DSP firmware and closed-loop applications. Performance specifications will be documented in due course, on the project website here.

As ever, it should be noted that any examples shown may not necessarily be the best or most complete solution.

Calibration Aspects Considered in this Article

  1. Scenarios for self-calibration, self-test
  2. The basic reference elements
    • Voltage, Frequency & Resistance 
  3. Capability to measure and to generate signals
    • ADC & DACs
  4. Basic Parameters for check and calibrate
    • ADC Chain Input offset
    • ADC Chain gain
    • ADC Chain Input Noise
    • DAC Chain Output offset
    • DAC Chain gain
    • Various Mode balances and trims
  5. External Confirming measurements
  6. Discussion


1. Scenarios for self-calibration, self-test

The project hardware is a modestly complex piece of instrumentation class mixed-signal electronics which is aimed at low-latency closed-loop applications. Many of the devices used are at the top-end of the device family specifications. There are a variety of modes for both normal operation and test & calibration use.

So, to set the scene, here is a table describing various test and calibration scenarios.


ItemUse Case
What functionality do we need ?
Time to execute
1Test and evaluation of 1st unit
  • Write settings and values to all devices
  • Read from ADCs
  • Some automation of setting and reading
Don't ask.
2Test of subsequent prototype units
  • A collection of semi-automated mode and signal settings and basic measurements
Open ended
3Production Test 
  • A collection of semi-automated mode and signal settings and basic measurements
a minute or two ?
4
Unit Self-Calibration test with external validation (occasional check)
  • A fully-automated two channel calibration sequence with result logging to disk with a few validating measurements using external references
a minute or two ?
5
Unit Self-Calibration test
  • A fully-automated two channel self-calibration sequence with result logging to disk
a minute or two ?
6Unit Routine Self-Test
  • A fully-automated two channel self-test of the unit loaded with its stored calibration values.
< a minute ?
7Unit configure for use
  • Apply calibration settings and user parameters for use
< 100ms ?

Table 1. Test and Calibration Scenarios

Item 1 - The need to test and evaluate the 1st unit

  At this point the design operation(apart from simulation) and manufacturing processes were unproven and with some 758 components, devices and sub-modules on the pcb, there was a lot of testing to do. Additionally, there was no code in the FPGA or anywhere else to allow the engineer to exercise the devices, toggle the relays and start evaluating functionality and performance.

Plan

 Broadly speaking, the plan was to write a minimal amount of FPGA code to access all the connected devices on the PCB and then provide a command and control link back to a PC to allow higher level functionality to be written in C#.

In this way, the writing of low-level device drivers in the FPGA, high-level engineering and user interfaces on the PC and the evaluation of the unit hardware could all proceed hand-in-hand.

After that, further extension and automation using the C# application could provide optimized sequences once the 1st unit had been evaluated and proven.


2. The basic reference elements

Voltage

   We require a stable and precise dc voltage so that we can calibrate the ADCs. For this we will use the buffered reference voltage available on the AD5752R trim control DAC. The relevant specification is shown below.

AD5752R DAC Voltage reference Specification

Fig 1. AD5752R DAC, Voltage Reference Specification

Frequency/Timing

  We need a stable and precise frequency reference to enable accurate time-related signal generation and measurement. The FPGA daughter board has a 24MHz Clock module SIT9201AC-S3-XXE-24.000000G.

Fig 2. SIT9201AC-S3-XXE-24.000000G Clock Specification -50/+50 Part

At +-50ppm this is not a particularly impressive part, but it is fine for initial evaluation. If a higher precision/stability part is required, it can be added onto the spare bank of power and I/O pins on the FPGA daughter board.

Resistance

100 ohm and 5k precision resistors are provided as reference loads for closed-loop applications. These allow closed-loop gains to be measured and can be used in the Abitrary Circuit Emulation application to verify the emulation performance.

Fig 3. Precision Load Resistor Specifications



3. Capability to measure and to generate signals 

ADC - LTC2387-16

  A measurement chain consisting of a compensated potential divider, a buffer amplifier and an ADC driver feeds a LinearTech/ADI SAR 16bit ADC in each channel to provide a measurement capability. The potential divider can be connected to 0V dc, the 2.5V dc reference or the channel I/O port.

The limit of device operation is 15Msps.

DAC - LTC1668

 The main signal generation capability is provided by a LinearTech/ADI 16bit DAC which provides a differential current to a dual op-amp and then on to the main port amplifier.

The limit of device operation is 50Msps.


4. Parameters for check and calibrate

ADC operation for calibration - general notes

  The following parameters and methods were used to gather ADC signal records for calibration use.

  • ADC conversion - initiated by the FPGA driving the ADC LVDS CVN Clock at 2Msps
  • ADC 16bit data - ADC LVDS receive data is clocked into a double data rate (DDR) decoder and stored into a record buffer.
  • ADC record - is 1024 samples * 0.5us = 512us duration
  • ADC Test Pattern - a control is provided to force the ADC to provide a known test pattern

The Schematic for the FPGA ADC management arrangement is shown below


Fig 4. Schematic of the present FPGA ADC system, including DDR decoder, Offset removal, sample store and timing manager state machine.

ADC - Known Test Pattern

The LTC2397-16 has a test mode pattern which in the 2 Lane LVDS  receive mode format is 0xCC3F Hex or -13249 signed decimal.

Here is the result of a 1024 ADC sample read record.

Fig 5. 1024 Samples with the ADC set to test pattern mode.

ADC Chain - Input Offset

  The ADC measurement signal chain is connected to 0V via a suitable termination.

In this case the average ADC chain offset is around 189 counts out of a full scale range of -32768 to +32767


Fig 6. 1024 Samples with the ADC chain connected to 0V via a suitable termination

  ADC Chain - Gain

     The ADC measurement signal chain is connected to 0V and 2.5V low impedance reference voltages and the V/bit calibration factor is then calculated. This next screen shot shows a confirming measurement of the 2.5V reference voltage after the 2 ADC calibration factors have been applied.


Fig 7. 1024 Samples with the ADC chain connected to the 2.5V dc reference

ADC Chain - Noise floor test

       The ADC measurement signal chain is connected to the 0V reference and a 32768 point sample record is collected. After removal of any dc the record is processed by an FFT with a Blackman-Harris window and scaled such that a sine-wave of full-Scale amplitude would be 0dB.


Fig 8. 0V Reference noise floor measurement using a windowed 32768 point FFT with 0dB representing the ADC full-scale amplitude

and the associated time-domain noise floor signal showing a maximum noise envelope of -8 to +3 counts.


Fig 9. 0V Reference time domain noise measurement

DAC Chain operation for calibration - general notes

  The following parameters and methods were used to generate DAC signals for calibration use.

  •  DAC arrangement - There is 1 main DAC per channel and 1 secondary DAC for use in some applications. The main DAC provides a differential current to a dual op-amp and then on to the main port amplifier.
  •  DAC Data - because of a limitation on I/O pins on the FPGA daughter card, a 16bit data bus is used and 3 of the 4 DACs have latches to allow the each DAC to hold its data from the DAC data bus. The 4th DAC is connected directly to the DAC data bus. When all 4 DACs have the correct data presented, a conversion signal can be applied.
  • DAC conversion - initiated by the FPGA driving each of the 4 DAC clock signals simultaneously or independently at a rate of 8Msps.
  • DAC data - is a combination of a DC/Offset value, a signal generator value and a value computed by the feedback controller DSP block.

DAC Chain - Offset calibration

  By setting various modes, setting a couple of trial DAC values and using the calibrated ADC to measure the resulting port voltage levels, we can determine the DAC dc trim values to ensure the port output is sensibly zero in it's quiescent state, as shown in the measurement below. The Y scale is +-3mV, measured value is < 1mV.

 

Fig 10. Measurement of port quiescent voltage after the DAC offsets have been applied.

DAC Chain - Gain calibration

  By setting various modes, setting a couple of trial DAC values and using the calibrated ADC to measure the resulting port voltage levels, we can determine the DAC V/bit gain values to allow the setting of precise dc voltage levels. The following screen shot shows the measurement summary for setting a value of +1Vdc after DAC chain offset and gain calibration factors have been applied.

Fig 11. Measurement after DAC chain calibration of the port voltage for a +1Vdc setting.

Various Mode balances and trims

Further calibration measurements are taken to calibrate additional circuitry that might be used for more complex architectures.


5. External Confirming measurements

The following table shows a set of results for various set voltages. For each set voltage the calibrated ADC measurement and a reading from an independent DVM connected to the User Port are taken.


Set VoltageADC Chain Self-MeasurementError(mV)External DVM Measurement
Error(mV)
-4-4.0018-1.8-4.002-2
-2-2.0011-1.1-2.001-1
-1-1.0009-0.9-1.001-1
-100m-100.1m-0.1
-100.4m-0.4
0-0.0002-0.2-0.0005-0.5
+100m99.8m-0.299.5m-0.5
10.9999-0.10.9997-0.3
22.0004+0.42.001+1
43.9992-0.84.0000

Table 2. Set Voltage, ADC Chain self-measure, External DVM measure and mV errors

Fig 12. Fluke 8060A DC Voltage specifications.


6. Discussion

DAC chain DC Voltage setting and ADC chain measurement accuracy

The self-measurement is always within +-1mV of the DVM reading, and of course the self-measurements only take only 512 us compared to 1second taken by the DVM.

In simplistic terms it looks reasonable to claim a setting and measuring capability of +-0.1%+-1mV for the testing so far and there is always the possibility to refine the calibration processes further. To take into account the DAC reference error and apply further averaging.

ADC Chain Noise floor

 At around -120 dB  when measuring the 0V reference, the noise floor is a bit higher than the LinearTech/ADI published ADC specification of around -128 dB. But, the present design does not yet have any significant filtering and the measurement chain consists of a medium impedance resistor divider followed by a wide-band buffer op-amp and an ADC driver in an open electrical environment. This probably compares to an optimized laboratory measurement for the vendor data. Pending optimization of the measurement chain filtering and possible decimation filtering, the -120 dB Noise floor with no discernible spurious components is more than acceptable.




Fig 13. LinearTech/ADI LTC2387-16  and the ACG Project, 32k FFT Noise floor

Thank you for your interest.

Next up - Will be Signal  Generators.
Previous post by Steve Maslen:
   Feedback Controllers - Making Hardware with Firmware. Part 5. Some FPGA Aspects.

Comments:

[ - ]
Comment by jms_nhDecember 4, 2017
The ADC measurement signal chain is connected to 0V via a suitable termination.
Don't assume that ADC voltage offsets are always positive; they can be negative, in which case a 0V input will be saturated as an ADC count of zero.
[ - ]
Comment by MixedSignalDecember 4, 2017

Hi jms_nh,

 Thank you for my 1st comment. All the calibration calculations are based on signed maths and the ADC chain is designed to accommodate a -7.5V to +7.5V  i/p range and to produce an ADC output code in the range of -32768 to +32767. The ADC Chain offset correction is also applied as a signed value.

No assumptions are made on either the sign of the offset or the sign of the gain constant. One thing I am guilty of is referring to ADC and DAC offsets and gains when in fact I really mean the ADC chain and the DAC chain rather than the ADC and the DAC in isolation. I will update the blog to clarify that.

In fact, 1 of the 2 unit channels has a +ve offset and the other a -ve offset and all is processed ok.

 

[ - ]
Comment by jms_nhDecember 4, 2017
ah -- ok, I missed that; I thought you were using a unipolar AD.

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