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Sampler is an 8-channel, 16-bit ADC EEM with an update rate of up to 1.5MSPS (all channels simultaneously). It has low-noise differential front end with a digitally programmable gain, providing full-scale input ranges between +-10mV (G=1000) and +-10V (G=1).

Note that this page describes the forthcoming Sampler v2.0, rather than the current (soon-to-be-obsolete) v1.1.

Overview

• Current hardware revision: v1.1 (first prototype), v2.0 forthcoming
• Cost: €500 (estimate)
• Width: 4HP
• Channel count: 8
• Resolution: 16-bit
• Sample-rate: up to 1.5MSMs (per channel in basic mode, or simultaneously on all channels in fast mode)
• Bandwidth: 200kHz -6dB bandwidth for G={1, 10, 100}, 90kHz for G=1000
• Input ranges: +-10V (G=1), +-1V (G=10), +-100mV (G=100), +-10mV (G=1000)
• DC input impedance:
  • Termination off: 100k from input signal and ground connections to PCB ground
  • Termination on: signal 50Ohm terminated to PCB ground, input ground shorted to PCB ground
• ADC: LTC2320-16
• PGIA: AD8253
• EEM connectors: power and digital communication supplied by a 1 EEM connector in basic mode or 2 EEM connectors in fast mode.
• Power consumption: ?

Modes of operation

Sampler can be operated either in basic mode as a standard SPI EEM using a single EEM connector (EEM0), or in fast mode via a source-synchronous LVDS interface and 2 EEM connectors (EEM0-EEM1).

In basic mode, the channels must be read out sequentially, in increasing order. The maximum SPI clock is limited by the round-trip delay in the cabling used to connect Sampler to the master. With a 20 MHz SPI read clock (25ns round trip plus setup), we get one sample one each channel per 6.9us (30ns tCONVH + 450ns tCONV + 8 channels * 16 bits * 50ns).

Fast mode allows all ADC channels to be read out simultaneously at 1.5MSPs via a source-synchronous interface. This is used in the Kasli-Sampler-Urukul "NU" laser intensity servo.

EEM connector usage:

Pin	EEM0	EEM1 (fast mode only)
0	ADC-SCK	ADC-CLKOUT
1	NC	ADC-SDOB
2	ADC-SDOA	ADC-SDOC
3	ADC-CNV	ADC-SDOD
4	PGIA-SCK	NC
5	PGIA-MOSI	NC
6	PGIA-MISO	NC
7	PGIA-CS	NC

"PGIA" refers to the front-end programmable-gain instrumentation amplifiers, which are controlled over SPI via a shift register.

Tests and characterisation

To do: check power handling of termination resistors!

Tests performed using Sampler Rev 1.1 prototype hardware. See issue 226 for further details. Further tests will be done on the Rev 2 hardware.

Unless stated otherwise, all measurements use:

• 200kHz ADC clock
• Input termination off for signals connected directly to a voltage source, termination on for "floating" inputs
• PGAI G=1

Bandwidth

• Drive an input (termination off) from a signal generator.
• Scan the signal frequency to measure the -6dB frequency on the ADC.
• The sample rate was 125kSPS, so the signal frequency was above the Nyquist frequency for most gain settings.

Small-signal bandwidth

• Signal was nominally -28dBFS=1Vpp/G (there was a slight error due to the ADC reference gain being incorrect in this hardware revision).

Gain	Small-signal BW (kHz)
1	206
10	207
100	195
1000	87

Large-signal bandwidth

• We made the following measurements, which confirm that the large-signal bandwidth is the same as the small-signal bandwidth to within the measurement accuracy for gains of 1 and 10.
  • G=10, 1Vpp=-8.1dBFS: 213kHz
  • G=10, 2Vpp=-1.2dBFS: 212kHz
  • G=1, 10Vpp=-8.1dBFS: 214kHz

Large-signal settling time

• This measurement is still "to do"; we didn't end up measuring the settling time, as we didn't have a sufficiently fast-settling voltage source to hand.
• Apply 1kHz, 50% duty-cycle square-wave to input
• Measure settling time as a function of input signal level

G=1

Input level	to 0.1% (us)	to 1 LSB (us)
22mVpp	?	?
220mVpp
2.2mVpp
10Vpp

G=1000

...

Noise floor

• Sample rate for this measurement was 200kHz, measurement bin width was 3Hz (65k samples)
• We measured the noise spectrum both for a terminated input (0V input) and (using batteries as a low-noise voltage source) for signals close to full-scale. No significant difference observed between the two cases. Data below are all for terminated (0V) inputs.
• No spurs seen above the noise floor at any frequency (e.g. no SMPS spurs seen)
• Noise spectrum is white once DC offset was subtracted
• Measure noise floor as a function of gain

gain	LSB RMS	µV RMS	nV/rtHz
1	1.3	420	TBD
10	1.4	44	TBD
100	3.6	11	TBD
1000	14.3	4.4	TBD

Channel-channel cross-talk

DC cross-talk measurement:

• Input A: apply +-17V DC, termination off. NB this voltage is clipped by the protection diodes, but was chosen to really stress-test the design.
• Input B: termination on measure signal level
• With input B G=1, voltage applied to input A makes <1LSB change in level measured at input B. DC cross-talk is thus <96dB.
• To do: re-measure DC cross-talk at G=1k

AC cross-talk measurement:

• Gain was the same on all channels.
• Apply 50kHz, 10Vpp (-8.8dBFS) signal to one channel. NB this saturates the PGIA for G>1!
• Measure signal on the adjacent channel as a function of PGIA gain
• Termination is on for the driven channel, but off for all other channels
• Note that the lack of dependence of the cross-talk on PGIA gain suggests that the cross-talk is dominated by pickup before the PGIA -- most-likely in the BNC connector.

Gain	Measured signal (dBFS)	Cross-talk (dBc)
1	-87	-79.2
10	-67	-78.2
100	-46	-77.2
1000	-29	-81.2

• With the termination on for all channels, we find:

Gain	Measured signal (dBFS)	Cross-talk (dBc)
1	<-130	<-121
10	-111.5	-123
100	-93.7	-124.9
1000	-72.2	-123.4

Common-mode rejection ratio (CMMR)

• Short input ground and signal together for one channel
• Apply sinewave to input signal/ground
• Measure the signal level seen by the ADC

G=1, signal ~2Vpp (-22dBFS)

Freq [kHz]	measured signal level [dBFS]	CMRR (dB)
0.01	<-120	>-98
0.1	< -120	>-98
1	<-120	>-98
10	-109	- 87
100	-87	-55
1000	-105	-83
10000	-108	-85

G=100, signal ~2Vpp (+18dBFS)

Freq [kHz]	measured signal level [dBFS]	CMRR (dB)
0.01	<-110	>-118
0.1	-98	-116
1	-88	- 106
10	-70	- 88
100	-50	- 68
1000	-80	- 98
10000	<-100	>-118

Harmonics

• 25kHz signal applied to input
• measure the level of the second harmonic (50kHz) and third harmonic (75kHz) as a function of input signal level.

Input level	Second harmonic	Third harmonic
0.1Vpp (-48dBFs)	-51dBc
1Vpp (-28dBFs)	-69dBc
10Vpp (-8dBFs)	-58.8dBc