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https://github.com/lucask07/open_covg_daq_pcb

DAQ (ADCs and DACs) PCB for the digital ion channel amplifier project.
https://github.com/lucask07/open_covg_daq_pcb

data-acquisition hardware

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DAQ (ADCs and DACs) PCB for the digital ion channel amplifier project.

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README 

          
# Real-Time Feedback DAQ Board



## Project Description



This repository houses the KiCad schematic, layout, and BOM for a general-purpose data acquisition board (DAQ) that enables real-time feedback with latencies of < 2 us by direct interfaces of ADCs and DACs to the [XEM7310](https://opalkelly.com/products/xem7310/) OpalKelly FPGA module. This DAQ board is designed for a digital feedback implementation of a clamp amplifier for the cut-open vaseline gap (COVG) electrophysiology method and may also find a use for other scientific studies requiring low latency control. 



The **schematic is [here](docs/covg_daq_v2.pdf)** and a 3d rendering of the board is shown below: 





**Contact**: Lucas Koerner at the University of St. Thomas, School of Engineering using the email: koerner dot lucas at stthomas.edu.



## Board Description



### Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs)

- **High-speed ADCs [AD7961 ADC @ 5 MSPS](https://www.analog.com/media/en/technical-documentation/data-sheets/AD7961.pdf) (x4)**: 16-bit differential input ADC with LVDS data interface to the FPGA, one cycle latency. Input must be differential with a common-mode voltage that matches the common-mode output of the part (~2.5 V).

- **General purpose ADC [ADS8686](https://www.ti.com/lit/gpn/ads8686s) (x1)**: 16 channels, maximum of 1 MSPS, SPI interface. The analog input range is programmable to +/-2.5 V, +/-5V, +/-10V. 8 channels are available on the pin-header J9; the other 8 channels are routed to the HDMI daughtercard connectors. 

- **High-speed DACs [AD5453 DAC](https://www.analog.com/media/en/technical-documentation/data-sheets/AD5450_5451_5452_5453.pdf) (x6):** 14-bit, multiplying R-2R ladder with 100 ns settling time. These fast DAC outputs connect to the HDMI daughtercard connecters and have programmable gain with full-scale bipoloar ranges of +/- 15V, +/- 5V, +/- 2 V, +/- 500 mV, +/- 200 mV.

- **General purpose DAC [DAC80508](https://www.ti.com/lit/ds/symlink/dac80508.pdf?ts=1624161469392&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDAC80508) (x2):** 8-channel, 16-bit, voltage output with 5 us settling time. The unipolar DAC outputs are converted to +/-7.5 V bipolar outputs before connecting to the HDMI daughtercard connectors. 

Pin header J11 has x8 bipolar outputs, x10 0-5V unipolar direct DAC output signals, and x2 "calibration" outputs which can be programmed to be the output of a current source (Howland) driven by the DAC.  



### Digital Interfaces 



**At connector J15:**



- Digital I/O level-shifted to 1.8 V levels (x6). (signals DNO[0-5]) 

- Digital I/O level-shifted to 5.0 V levels (x6). (signals UPO[0-5])

- Digital I/O at 3.3 V levels (x4). (signals GPIO[0-3])

- I2C interface at 1.8 V (1V8\_SCL, 1V8\_SDA)



**At connector J10:**



- General LVDS I/O 4 differential pairs at 2.5 V. Can also be used as 8 LVCMOS I/O. Note the routing of these LVDS pairs is only very roughly impedance controlled. Performance at the highest frequencies will need evaluation. 



**At SMA connectors J16 and J17:**



- J16 and J17, 3.3V LVCMOS routed to multi-region clock capable pins. Potential use is to synchronize with other systems. 



**At QWIIC connector, J18:**



- The [Sparkfun QWIIC connector](https://www.sparkfun.com/qwiic) has 4 pins. 3.3 V power, I2C (SDA and SCL), and Gnd. This is a standardized interface to I2C-based sensor boards.  



### Power

An input voltage of 6 to 7V is supplied at barrel connector J12 ([PJ-002BH](https://www.digikey.com/en/products/detail/cui-devices/PJ-002BH/408447)) with a maximum rating of 5 A. 



The power system is as follows: 



- Dual DC-DC converter to +/-16.5 V 

	  - The +/-16.5 V is regulated down to +/-15 V (linear)

- Regulator to 5 V (linear)

- Regulator to 3.3 V (linear)

- Regulator to 2.5 V for LVDS bank on FPGA (linear)

- Regulator to 1.8 V (linear)

- Voltage reference for 2.048 V. 



All power nets are connected to a screw terminal block connector to allow for monitoring or override of the on-board regulated voltages.  



### FPGA capabilities 

The [XEM7310](https://opalkelly.com/products/xem7310/) OpalKelly FPGA module has a Artix7 FPGA (XC7A5T-1) with a USB 3.0 interface to a host computer (transfer rates up to 30 MiB/s). The module has 1-GiByte of DDR3 to enable burst data captures or burst data transfers. 



## Daughtercard Channels (x4)



The DAQ board has four connections to support analog daughtercards. The daughtercard connections are standardized and use an HDMI connector. For the COVG electrophysiology amplifier, these channels are the bath clamp, guard clamp, V1/I top clamp, and (optionally) the V2 monitor. Future experiments could use one or up to all of these daughtercard connections. 



### Channel connection definition 

The generic HDMI-A cable pinout is defined as follows with 15 (non-shield) pins used:



- **Power**: (3 pins) +5V, +15V, -15V

- **GND**: (1 pins) 

- **Analog inputs** to ADCs: (4 pins) 

     - OUT\_P, OUT\_N differential pair (x1) to 5 MSPS ADC 

     - single-ended out AMP\_OUT, CAL\_ADC each connect to an individual channel of 1 MSPS ADC (ADS8686) 

- **Analog reference output**: (1 pin) buffered ADC common-mode voltage (nominal 2.5 V) from the AD796x.

- **Digital outputs** from DACs: (3 pins) 

	- Fast DAC1 (CMD)

	- Fast DAC2 (CC) on channels 0 and 1. From general-purpose DAC on channels 2 and 3.  

	- From general purpose 5 us settling time DAC (DAC8050). Either voltage source or current source (Howland current pump), programmable (CAL_DAC).

- **I2C Digital control**: (2 pins) SDA, SCL (@ 3.3 V levels). Pull-up resistors (3.3 kOhm) are placed on the DAQ board. A common approach is for the daughtercards to use an I/O expander (such as the TCA9555) to generate many control signals from the 2-wire I2C interface. 

- **Utility**: (1 pin) Resistor jumper sets analog in, analog out, or digital I/O (@ 3.3 V levels). The default stuffing option is to connect this pin to the general-purpose ADC (ADS8686). 



| Pin | HDMI name    | Generic Daughtercard     | Notes     |

|-----|--------------|--------------------------|----------------------------|

| 1   | Data2+       | fastDAC1     | ~ 1 us settling time. Programmable full-scale range.                                                                          |

| 2   | Data2 shield | gnd          |                                                                            |

| 3   | Data2-       | Analog/GPIO1 |  DAC8050, 16 bit, 5us settle, 5 V supply                                                                         |

| 4   | Data1+       | fastADC+     |  Analog output to AD7960/AD7961. Differential low-pass filter on DAQ board. Common mode voltage must match 2.5 V.                                                                          |

| 5   | Data1 shield | gnd          |                                                                           |

| 6   | Data1-       | fastADC-     |  Analog output to AD7960/AD7961. Differential low-pass filter on DAQ board. Common mode voltage must match 2.5 V.                                                                          |

| 7   | Data0+       | fastDAC2     |  ~1 us settling time. Programmable full-scale range.                                                                          |

| 8   | Data0 shield | gnd          |                                                                           |

| 9   | Data0-       | Analog/GPIO2 |  Analog output to ADS8686 1 MSPS. Chip inputs includes a programmable PGA for full-scale range up to +/-10V                                                           |

| 10  | Clock+       | 15V          |  linear regulator, 75 mA max per channel                                                                         |

| 11  | Clock shield | gnd          |                                                                            |

| 12  | Clock-       | VCM          | reference voltage from the AD796x buffered on the DAQ board, nominal 2.5 V                                                                          |

| 13  | CEC          | -15V         |   linear regulator, (estimate is 75 mA max per channel with all 4 channels connected)                                                                          |

| 14  | Reserved     | selectable auxillary         | Resistor jumper sets analog in, analog out, or digital I/O (@ 3.3 V levels). Default stuffing option is to connect to the general purpose ADC (ADS8686).   |

| 15  | SCL          | SCL          |  I2C clock. Pull-up resistor on DAQ board, 3.3 V levels.                                                        |

| 16  | SDA          | SDA          |  I2C data. Pull-up resistor on DAQ board, 3.3 V levels.                                                            |

| 17  | Gnd          | Gnd          |                                                                            |

| 18  | 5V           | 5V           |  Power from linear regulator (target 200 mA max per channel with all 4 channels connected)                                                                       |

| 19  | HPD          | Analog/GPIO3 | Analog output to ADS8686 1 MSPS. Chip inputs includes a programmable PGA for full-scale range up to +/-10V         



### Example Daughtercard Pinout 



The pin names shown in the image below are an example daughtercard interface (COVG bath-clamp). 





## FPGA Setup 

Because of the LVDS connections to the AD7961 FPGA bank 34 needs to be at 2.5 V. To do so remove ferrite bead (FB8) from the OpalKelly module. See the section "Setting the Expansion Connector I/O Voltages" [here](https://docs.opalkelly.com/xem7310/expansion-connectors/).



## For Board and FPGA Designers



### KiCAD Setup and Required Libraries 



Symbols and footprints custom to this design (and the COVG project in general) are housed in the [covg-kicad-lib repository](https://github.com/lucask07/covg-kicad-lib). This library should be a project specific library in your KiCAD *Symbol Library Management* and *Footrpint Library Management*.  



When at all possible, footprints are used from the KiCAD libraries. 



### Impedance control of LVDS lines 



Impedance control of the LVDS lines to the AD796x was done using the [6-layer stackup from PCBWAY](https://www.pcbway.com/multi-layer-laminated-structure.html) with board thickness of 1.6 mm and copper (outer/inner) thickness of 1oz/1oz. The design has the impedance-controlled LVDS signals routed on the top or bottom layer (and therefore a microstrip referenced to layer 2 or layer 15). The differential pairs are skew matched on both the DAQ board and the FPGA modules. The (inter pair) signals for each AD796x have effectively a half clock cycle of slack for skew so are reasonably but not precisely length matched. The DCO signal (AD796x clock out) is routed to a clock capable pin on the XEM7310. 



### Power Planes on the PCB (6 layer board)



* Top: routing (preferred left/right)

* Route 2: GND

* Route 3: 3.3V and 1.8V 

* Route 14: +15V, -15V

* Route 15: 5V, 2.5V, and 6V input

* Bottom: routing (preferred up/down)



### Routing and autorouting methods

The AD796x impedance controlled differential pairs were routed using the KiCAD differential pair tool and the power planes were drawn in KiCAD to match the location of the various power domains. Parts were placed manually and the 'Replicate layout' KiCAD plug-in was used to duplicate the placement of identical blocks. Then the board was exported to use the tool *freerouting* to route most of the board. After *freerouting* it was found to be critical to confirm that inner layer routing did not destroy the integrity of the power planes. 



### FPGA pinout



- [pinout](https://www.xilinx.com/support/packagefiles/a7packages/xc7a75tfgg484pkg.txt) of the Artix7



- [XEM7310 pinout](https://pins.opalkelly.com/pin_list/XEM7310)



### Manufacturer's Part Numbers and Bill of Materials



The KiCAD schematic uses a field **Manf#** to specify the precise manufacturer's part number for component sourcing. This also allows the (great) KiCAD External Plugin KiCost to be used to check board component cost and availability. 

One complication is that the OpalKelly XEM7310 exports as one BOM line item. Ultimately, though, the board must be populated with two Samtec [BTE-040-02-F-D-A](https://www.digikey.com/en/products/detail/samtec-inc/BTE-040-02-F-D-A/6677927) high-density connectors at silk-screen labels MC1 and MC2. See the part [position file](gerbers/covg_daq_v2-all.pos): 



`

MC1       Connector_SMD        BTE-040-02-F-D-A                       108.6849    33.1811  180.0000  top `

`MC2       Connector_SMD        BTE-040-02-F-D-A                       108.6849    73.1911  180.0000  top

`



The [bill of materials files are here](bom/). 



Note that the "chip shortage of 2020/2021" has made component sourcing complicated and we hope this bottle-neck eases in the near future. 



### Assembly Guidance

The prototype boards were assembled by PCBWAY. The information provided to the assembly house is in the [assembly folder](assembly/).



### Board Issues

Please see the issues section of the GitHub repository. The most significant considerations are:



1) The fast DACs must be flipped (upside down): pin 1 -> pin 8, pin 8 -> pin 1, etc.

2) The switching power supply does not source enough current so has been removed. 3 voltages are provided externally: 7 V, +16.5 V, -15 V. 

3) The negative -15 V output regulator demonstrated low-level oscillations so has been removed and is bypassed. 



### Other board design notes 



**Unconnected level translator pins** [unused inputs](https://e2e.ti.com/support/logic-group/logic/f/logic-forum/788628/faq-what-should-be-done-with-unused-i-o-pins-of-the-level-translator-devices)

The level translator used can leave unused pins disconnected.



**Barrel jack**: The Opal Kelly FPGA barrel connector is 2.1 mm ID / 5.5 mm OD while the DAQ board power jack 2.5 mm ID / 5.5 mm OD are different sizes. 



Why do the barrel jacks have 3-pins and what to do with the 3rd pin? [3-pin DC jack](https://electronics.stackexchange.com/questions/90529/what-to-do-with-third-contact-in-dc-barrel-plug-with-only-two-input-contacts)



**HDMI** cable casing is 20 mm so at least 25 mm pitch is needed between connectors. 



## Companion Software and Firmware



* The FPGA code is hosted at [https://github.com/lucask07/covg_fpga](https://github.com/lucask07/covg_fpga)

* The Python host computer to FPGA code is available at [https://github.com/Ajstros/pyripherals/](https://github.com/Ajstros/pyripherals/)



## Funding 

Research reported in this publication was supported by the National Institute Of Neurological Disorders And Stroke of the National Institutes of Health under Award Number R15NS116907. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.



## Copyright and Licence 



Copyright 2021 Lucas J. Koerner.



See the Git repo for License details.








## Board Fabrication and Assembly

Boards were fabricated and assembled by PCBWAY (https://www.pcbway.com/). 







PCBWAY successfully completed turnkey assembly (found and ordered parts) and carefully reviewed the design and assembly files.