1. ADC and TDC Implemented Using FPGA
Jinyuan Wu, S. Hansen and Z. Shi
Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
FPGA ADC and TDC
Intrinsically, FPGA is a digital device. However, with suitable use of the FPGA resources, it is possible to use FPGA to digitize multi-channel analog waveforms. The digitized waveforms can be directly processes in the FPGA. There are several possible schemes of digitizing analog signals. One of the schemes we used in our FPGA ADC study is based on the ramping-comparing approach.
In today’s FPGA devices, differential input buffers are good comparators within a sufficiently large range of input voltage levels, since they are designed to be compatible with various differential signaling standards. Comparator-based ADC schemes can be implemented with FPGA.
In our tests, the analog inputs are directly connected to the FPGA input pins. A passive RC network is connected to the FPGA output pins so that a periodic reference voltage ramp can be generated. The differential input buffers are used as comparators to generate logic transitions inside the FPGA when the reference voltage ramps across the input voltage levels. The transition times are digitized by the TDC block implemented in the FPGA. Since the period, the RC network parameters and the starting time of the ramps are known the input voltage levels can be derived from the transition times. (In some references, the single-slope scheme is mistakenly referred as Wilkinson ADC that is based on dual-slope principle.)
A key functional block, Time-to-Digit-Converter (TDC) is needed in FPGA. The TDC we used in this work is multi-sampling scheme with quad clock. In our TDC design, the four samples are transferred into a bit pattern in a single clock domain immediately and only one set of edge detect, pulse filter and count latch circuit is used. The meta-stability is limited at the sampling stage only and in fact, the meta-stability in sampling stage does no harm but carrying the input signal arrival time information. The decoding becomes very simple in our design. The detail is described in the paper.
The TDC in FPGA alone is already very useful. The TDC card designed for Fermilab MIPP upgrade project is documented in this paper.
The multi-sampling structure can have other applications. A deserializer circuit known as “Digital Phase Follower” (DPF) is also documented. Using DPF, any FPGA input can be used to receive serial data without needing dedicated deserializer that is only available in high-end FPGA families. The DPF can compensate input data phase drift not only due to cable temperature variation, but also due to crystal oscillator frequency difference between transmitter and receiver.
FPGA Direct Analog Signal Digitization
Multi-Sampling Based TDC
AMP &
Shaper
FPGA
TDC R1 C R2
VREF
AMP &
Shaper
ADC
FPGA
Encode c0
c90
c180
c270
Data In Q0 Q1 Q2 Q3 QF QE QD DV T0 T1 TS
Single Slope ADC: Ramping-Comparing
4Ch T1 V1 T2 V2 T3 V3 T4 V4
Internal Layout Inside FPGA
Single Slope ADC != Wilkinson ADC
TDC Bench Test Results
Test Result: BD3_19, Quasi-Linear VREF
Micro-processor address line driven by
45 MHz clock.
32 TDC bin = 1 clock cycle (0.69ns LSB)
Micro-processor data line.
Data out from different sources.
eZ80
RAM
Flash
FPGA
TDC
45MHz
Input Waveform
Input Waveform
& Reference Voltage
Overlap Trigger
FPGA
TDC 50
1000pF
100
VREF
The 96-Channel TDC Module
Raw Data
t = 59 ns, Reference Voltage
48CH TDC FPGA
Converted
48CH TDC FPGA
Data Concentration FPGA
Digital Phase Follower (DPF): A TDC-Like Deserializer
Test Result: BD4_22, Exponential VREF
It is used for serial communication between low-cost FPGA devices.
No dedicated clock-data-recovery (CDR) circuitry is needed.
It tolerates multi-crystal operations.
Input Waveform c0
c90
c180
c270 In
Multiple
Sampling
Clock
Domain
Changing b0 b1
Frame
Detection
Data
Out
Tri-speed
Shift
Register
Shift2
Shift0
was3
is0
SEL
was0
is3
Trans. Q0 Q1 Q2 Q3 QF QE QD
FPGA
TDC 50
150pF
100
VREF
FPGA X1 X2
t = 7.5 ns Reference Voltage
Raw Data
Small pulses are emphasized by the trailing ramp measurement.
The data measured by trailing ramp is much more smoother than the leading ramp for small pulses.
Converted