LADAR APD focal plane array(NB page) - Voxtel Inc.voxtel-inc.com/files/2012/07/LADAR-APD-focal-plane-array-NB-page.… · Features High gain, low excess noise single carrier multi-plication

Features

▪ High gain, low excess noise single carrier multi-

plication (SCM) linear mode avalanche photodi-

ode (APD) arrays

▪ APD gain up to M = 40

▪ Greater than 85% QE across the 950nm to

1650 nm spectral band

▪ Better than 200 ps time jitter

▪ Time stamp and amplitude sample of first three

or first and last pulse returns

▪ Constant fraction discrimination for low time

walk

▪ Better than 6 ns pulse pair resolution

The V7P-FFEXA is the highest sensitivity linear mode avalanche photodiode (APD) receiver available. It is optimized for highly precise LADAR measurements in complex real world environments where multiple pulse returns are used to see obscured targets. The V7P-FFEXA allows up to three time stamp and signal am-plitude data pair samples per frame and operates at rates up to 10 thousand frames per second (fps). The V7P series features Voxtel’s multi-gain-stage linear-mode InGaAs single carrier multiplication (SCM) APD, which can achieve an 6 photon noise equivalent input (NEI) sensitivity, when coupled to the receiver’s VX-804 readout integrated circuit (ROIC). Note: NEI=6 photons applies to a signal delivered to each pixel and assumes the use of a microlens. The key innovation of Voxtel’s SCM-APD is a method to increase the gain at which low-noise multiplication can be achieved. The SCM APDs are characterized by much lower excess multiplication noise than conven-tional linear-mode InGaAs APDs, which enables op-eration at higher avalanche gain and improves pulse detection efficiency (PDE) and lower false alarm rate (FAR). Excess multiplication noise is key to weak de-tection because it measures the weak statistical distri-bution of pulse amplitudes emitted by an APD operated at a given average gain. Additionally lower noise APDs emit a higher proportion of pulses exceeding a

LADAR APD Focal Plane Array32 x 32 Element LADAR Focal Plane Array

Model V7P-FFEXA: LADAR Focal Plane Array

Voxtel Literature No. RxP1-xJAF, Version date: 06/2012 ©Voxtel makes no warranty or representation regarding its products’ specific application suitability and may make changes to the products described without notice.

V7P-FFEXA Series LADAR APD Focal Plane Array

Voxtel, Inc., 15985 NW Schendel Avenue, #200, Beaverton, OR 97006, www.voxtel- inc.com, T 971.223.5646, F 503.296.2862

useful detection threshold. The SCM-APD arrays provide the high gain and the low excess noise necessary for high sensitivity LADARimaging when used with Voxtel’s VX-804 readout inte-grated circuit (ROIC), which operates with better than 200 ps time jitter (time precision) and 6 ns pulse pair resolution. Uniformity correction and failure protec-tion are implemented at the input node of each pixel. A digital-to-analog converter (DAC) programmed by static random access memory (SRAM) is used to in-dividually adjust the bias across the detectors’ pixels, and a fuse will protect the ROIC if a pixel fails short. After calibration the correct bias level is stored in each pixel’s SRAM. The 6 ns pulse-pair resolution is achieved by virtue of the pixel’s low noise GHz-class preamplifier, followed by a pulse detection circuit that uses twin compara-tors to validate and time the pulse return. One pro-grammable comparator checks to see if the input sig-nal crosses a detection threshold. Adjustment of this threshold allows the user to trade off pulse detection efficiency (PDE) for reduced false alarm rate (FAR); a lower threshold will allow the camera to detect a larger fraction of weak pulse returns, but also increase the FAR from preamplifier noise and dark current. The second programmable comparator sets the threshold level of the constant fraction discrimin-

LPF

ConstantFraction

Discriminator

Low-NoisePreamplifier

Fuse

PhotodiodeCommonCathode

Threshold

SRAM

DAC

OutputBuffer

OutputMux

AmpSample

RangeSample

Analog Ramp

Digital Code

Voxtel Literature No. RxP1-xJAF, Version date: 06/2012 © Voxtel makes no warranty or representation regarding its products’ specific application suitability and may make changes to the products described without notice.


2

ator (CFD), which is used to accurately measure the time of flight (TOF) of the pulse return. For pulse returns strong enough to exceed the programmed threshold of the first comparator, the CFD outputs a digital pulse that is timed to the CFD comparator. The output of the pulse detection circuit is used to control the sampling of both the range and amplitude data. When triggered by the CFD, the amplitude sam-pling block records the integrated signal level at the output of the preamplifier. The analog voltage level that is recorded by the range sampling block at the time of the trigger from the CFD represents a time stamp that can be mapped to a specific time-of-flight or target range by off-chip camera electronics. The VX-804 has memory locations in each pixel to store time and amplitude data of up to three pulse returns per frame. There are two modes for using this in-pixel memory. In one mode, the first three pulse returns are stored, and any subsequent returns are ignored. In the other mode, the first pulse return is stored, and subsequent returns “ping-pong” between the two remaining memory locations. The net effect is to store the first and last pulse return during the range gate. The next-to-last return is lost, because that memory location is overwritten; the circuit cannot know how many pulse returns it will receive, so in this mode, one memory location is always tracking the out-put from the signal chain, waiting for the ‘next’ return. The SCM-APD arrays have reduced detector re-set time, hence much less dead time, relative to the existing Geiger-mode single-photon-sensitive APD (SPAD) technology, allowing for better target detec-tion and reduced system size, weight, and power (SWAP). Due to their long reset times, Geiger APD pixels are blinded by obscurants and cannot resolve tar-gets closely spaced in range, reducing laser power efficiency by not recording target photons incidentat the aperture and increasing the number of laser shots which must be accumulated to image an ob-scured target. The aperture efficiency (number of

photons detected versus the number of photons at the aperture) of Geiger mode APDs is very low, which requires that the target information be accumulated statistically from a multitude of laser shots. High-gain linear mode APDs are characterized by pixel dead time 2-3 orders of magnitude shorter than Geiger mode APDs, and unlike Geiger mode APDs, they have the ability to measure signal amplitude informa-tion. These traits reduce laser power and number of shots required for image formation by maximizing the aperture efficiency of the system. Linear mode APD-based detectors with reduced pixel dead time greatly improve penetration of obscurants, and enable the sensor to resolve objects closely spaced in range. Fur-ther, linear mode APDs can measure signal amplitude, providing additional target information to support im-age processing and analysis.

B l o c k d i a g ra m o f o n e c h a n n e l o f t h e V 7 P- F F E X A’s p i xe l s i g n a l c h a i n




3

Pixa

l Ano

de B

ias

(V)

DAC Code

2.0

1.8

1.6

1.00 2 4 6 8 10 12 14 16

1.2

1.4

y = 1.1902 + 0.05015x R = 0.99777

L i n e a r f i t t o m e a s u r e d o u t p u t f r o m t h e 4 - b i t A P D p i x e l b i a s a d j u s t m e n t c i r c u i t i n a V X 8 0 4 p i x e l .

Vol

tage

Out

(mV

)

Input Charge (e-)

300

250

200

150

100

0 500 1000 1500 2000 2500 3000 3500 40000

50

y = -28.382 + 0.077043x R = 0.99801y = -21.097 + 0.073931x R = 0.99981

CFDLE

M e a s u r e m e n t o f a m p l i t u d e l i n e a r i t y u s i n g e i t h e r c o n s t a n t f r a c t i o n d i s c r i m i n a t i o n ( C F D ) o r l e a d -i n g e d g e ( L E ) d i s c r i m i n t a i o n .

Mea

sure

d Ti

me

Del

ay (n

s)

Input Charge (e-)

10.5

10

9.5

9

8.5

70 2000 4000 6000 8000 10000 12000

7.5

8

CFDLE

Ti m e w a l k a n d j i t t e r o f t h e V X - 8 0 4 m e a s u r e d f o r l a r g e i n p u t s i g n a l s , u s i n g e i t h e r c o n s t a n t f r a c -t i o n d i s c r i m i n a t i o n ( C F D ) o r l e a d i n g e d g e ( L E ) .

2400

2200

2000

1800

1600

1400

Ana

log

Ram

p O

utpu

t (m

V)

5 10 15 20 25Time (us)

CFDLE

y = 1058.6 + 59.011x R = 0.99998y = 1050.2 + 59.783x R = 0.99998

L i n e a r i t y o f t h e o n - c h i p a n a l o g t i m e s t a m p g e n -e r a t o r w a s m e a s u r e d t o b e 0 . 5 % .




4

Parameter Value

Application NIR flash LADAR

Detector Back-illuminated InGaAs SCM-APD

Nominal Operating Temperature 230K

Format 32 x 32

Pixel Pitch 36 µm

Optical Fill Factor 24% (without µ-lens) > 90% with microlens

APD Gain 10 - 40

Global Detector Bias Resolution 50 mV (sourced off-chip)

Independent Pixel Bias Resolution

(Uniformity Correction)1 V span; 67 mV resolution (sourced on-chip)

Maximum ROIC Noise 42 electrons

Noise-Equivalent Input (1064-1550 nm) 6 photons*

Amplitude Dynamic Range 8 bits

Temporal Dynamic Range

(Search Volume Depth)12 bits

Pulse Return Samples per

Frame per Pixel

first and last or first three per pixel

per laser shot, amplitude & TOF

Minimum Pulse-Pair Resolution 6 ns

Range Walk (not corrected for amplitude) 500 ps

Time-of-Arrival Precision (Jitter) 200 ps

Frame Rate 10 kHz

Specifications


* Assumes use of a microlens

Documents

LADAR APD focal plane array(NB page) - Voxtel Inc.voxtel-inc.com/files/2012/07/LADAR-APD-focal-plane-array-NB-page.… · Features High gain, low excess noise single carrier multi-plication