Datasheet AD8109AST, AD8109-EB, AD8108AST Datasheet (Analog Devices)

325 MHz, 8  8 Buffered Video

a

FEATURES
8  8 High-Speed Nonblocking Switch Arrays

AD8108: G = +1

AD8109: G = +2
Serial or Parallel Programming of Switch Array
Serial Data Out Allows “Daisy Chaining” of Multiple

8  8s to Create Larger Switch Arrays
Output Disable Allows Connection of Multiple Devices
Pin-Compatible with AD8110/AD8111 16  8 Switch

Arrays
For 16  16 Arrays See AD8116
Complete Solution

Buffered Inputs

Eight Output Amplifiers,

AD8108 (G = +1),
AD8109 (G = +2)

Drives 150  Loads
Excellent Video Performance

60 MHz 0.1 dB Gain Flatness

0.02%/0.02 Differential Gain/Differential Phase Error
= 150 )

(R

L

Excellent AC Performance

AD8108 AD8109
–3 dB Bandwidth 325 MHz 250 MHz
Slew Rate 400 V/s 480 V/s

Low Power of 45 mA
Low All Hostile Crosstalk of –83 dB @ 5 MHz
Reset Pin Allows Disabling of All Outputs (Connected

Through a Capacitor to Ground Provides “Power­On” Reset Capability)

Excellent ESD Rating: Exceeds 4000 V Human Body

Model

80-Lead LQFP Package (12 mm  12 mm)

APPLICATIONS
Routing of High-Speed Signals Including:

Composite Video (NTSC, PAL, S, SECAM.)
Component Video (YUV, RGB)
Compressed Video (MPEG, Wavelet)
3-Level Digital Video (HDB3)

PRODUCT DESCRIPTION

The AD8108 and AD8109 are high-speed 8 × 8 video cross­point switch matrices. They offer a –3 dB signal bandwidth
greater than 250 MHz and channel switch times of less than
25 ns with 1% settling. With –83 dB of crosstalk and –98 dB
isolation (@ 5 MHz), the AD8108/AD8109 are useful in many
high-speed applications. The differential gain and differential

Crosspoint Switches

AD8108/AD8109

FUNCTIONAL BLOCK DIAGRAM

SER/PAR

CLK

DATA IN

UPDATE

CE

RESET

AD8108/AD8109

8 INPUTS

phase of better than 0.02% and 0.02° respectively along with

0.1 dB flatness out to 60 MHz make the AD8108/AD8109 ideal
for video signal switching.

The AD8108 and AD8109 include eight independent output buff­ers that can be placed into a high impedance state for paralleling
crosspoint outputs so that off channels do not load the output bus.
The AD8108 has a gain of +1, while the AD8109 offers a gain
of +2. They operate on voltage supplies of ±5 V while consuming
only 45 mA of idle current. The channel switching is performed via
a serial digital control (which can accommodate “daisy chaining”
of several devices) or via a parallel control allowing updating of
an individual output without re-programing the entire array.

The AD8108/AD8109 is packaged in an 80-lead LQFP package
and is available over the extended industrial temperature range
of –40°C to +85°C.

D0 D1 D2 D3

32-BIT SHIFT REGISTER

WITH 4-BIT

PARALLEL LOADING

32

PARALLEL LATCH

32

DECODE

8  4:8 DECODERS

OUTPUT
BUFFER

64

G = +1,

SWITCH
MATRIX

SET INDIVIDUAL

OR RESET ALL

OUTPUTS
TO "OFF"

8

G = +2

ENABLE/DISABLE

*

A0
A1

A2

DATA
OUT

8 OUTPUTS

*Patent Pending.

REV. A

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] © Analog Devices, Inc., 2002

AD8108/AD8109–SPECIFICATIONS

(VS = 5 V, TA = +25C, RL = 1 k unless otherwise noted)

AD8108/AD8109 Reference

Parameter Conditions Min Typ Max Unit Figure No.

DYNAMIC PERFORMANCE

–3 dB Bandwidth 200 mV p-p, R

2 V p-p, R
Propagation Delay 2 V p-p, R
Slew Rate 2 V Step, R
Settling Time 0.1%, 2 V Step, R
Gain Flatness 0.05 dB, 200 mV p-p, R

0.05 dB, 2 V p-p, R

0.1 dB, 200 mV p-p, R

0.1 dB, 2 V p-p, RL = 150 Ω 80/50 MHz TPC 1, 7

NOISE/DISTORTION PERFORMANCE

Differential Gain Error NTSC or PAL, R

NTSC or PAL, R
Differential Phase Error NTSC or PAL, R

NTSC or PAL, R
Crosstalk, All Hostile f = 5 MHz 83/85 dB TPC 2, 8

f = 10 MHz 76/83 dB TPC 2, 8
Off Isolation, Input-Output f = 10 MHz, R
Input Voltage Noise 0.01 MHz to 50 MHz 15 nV/√Hz TPC 14, 20

DC PERFORMANCE

Gain Error R

= 1 kΩ 0.04/0.1 0.07/0.5 %

L

R

= 150 Ω 0.15/0.25 %

L

Gain Matching No Load, Channel-Channel 0.02/1.0 %

= 1 kΩ, Channel-Channel 0.09/1.0 %

R

L

Gain Temperature Coefficient 0.5/8 ppm/°C

OUTPUT CHARACTERISTICS

Output Impedance DC, Enabled 0.2 Ω TPC 18, 24

Disabled 10/0.001 MΩ TPC 15, 21
Output Disable Capacitance Disabled 2 pF
Output Leakage Current Disabled, AD8108 Only 1/NA µA
Output Voltage Range No Load ±2.5 ± 3V
Output Current 20 40 mA
Short Circuit Current 65 mA

INPUT CHARACTERISTICS

Input Offset Voltage Worst Case (All Configurations) 5 20 mV TPC 29, 35

Temperature Coefficient 12 µV/°C TPC 30, 36
Input Voltage Range ±2.5/±1.25 ±3/±1.5 V
Input Capacitance Any Switch Configuration 2.5 pF
Input Resistance 1 10 MΩ
Input Bias Current Per Output Selected 2 5 µA

SWITCHING CHARACTERISTICS

Enable On Time 60 ns
Switching Time, 2 V Step 50% UPDATE to 1% Settling 25 ns
Switching Transient (Glitch) Measured at Output 20/30 mV p-p TPC 16, 22

POWER SUPPLIES

Supply Current AVCC, Outputs Enabled, No Load 33 mA

AVCC, Outputs Disabled 10 mA

AVEE, Outputs Enabled, No Load 33 mA

AVEE, Outputs Disabled 10 mA

DVCC 10 mA
Supply Voltage Range ±4.5 to ±5.5 V
PSRR f = 100 kHz 73/78 dB TPC 13, 19

f = 1 MHz 55/58 dB

OPERATING TEMPERATURE RANGE

Temperature Range Operating (Still Air) –40 to +85 °C

θ

JA

Specifications subject to change without notice.

Operating (Still Air) 48 °C/W

= 150 Ω 240/150 325/250 MHz TPC 1, 7

L

= 150 Ω 140/160 MHz TPC 1, 7

L

= 150 Ω 5ns

L

= 150 Ω 400/480 V/µs

L

= 150 Ω 40 ns TPC 6, 12

L

= 150 Ω 60/50 MHz TPC 1, 7

L

= 150 Ω 60/50 MHz TPC 1, 7

L

= 150 Ω 70/65 MHz TPC 1, 7

L

= 1 kΩ 0.01 %

L

= 150 Ω 0.02 %

L

= 1 kΩ 0.01 Degrees

L

= 150 Ω 0.02 Degrees

L

=150 Ω, One Channel 93/98 dB TPC 17, 23

L

–2–

REV. A

AD8108/AD8109

TIMING CHARACTERISTICS (Serial)

Limit

Parameter Symbol Min Typ Max Unit

Serial Data Setup Time t
CLK Pulsewidth t
Serial Data Hold Time t
CLK Pulse Separation, Serial Mode t
CLK to UPDATE Delay t
UPDATE Pulsewidth t
CLK to DATA OUT Valid, Serial Mode t

1

2

3

4

5

6

7

20 ns
100 ns
20 ns
100 ns
0ns
50 ns

180 ns

Propagation Delay, UPDATE to Switch On or Off – 8 ns
Data Load Time, CLK = 5 MHz, Serial Mode – 6.4 µs
CLK, UPDATE Rise and Fall Times – 100 ns
RESET Time – 200 ns

CLK

DATA IN

1 = LATCHED

UPDATE

0 = TRANSPARENT

DATA OUT

t

1

0

t

1

1

OUT7 (D3)

0

2

t

3

t

7

t

4

LOAD DATA INTO
SERIAL REGISTER
ON FALLING EDGE

OUT7 (D2) OUT00 (D0)

TRANSFER DATA FROM SERIAL

REGISTER TO PARALLEL

LATCHES DURING LOW LEVEL

t

5

t

6

Figure 1. Timing Diagram, Serial Mode

Table I. Logic Levels

V

IH

RESET, SER/PAR RESET, SER/PAR RESET, SER/PAR RESET, SER/PAR
CLK, DATA IN, CLK, DATA IN, CLK, DATA IN, CLK, DATA IN,
CE, UPDATE CE, UPDATE DATA OUT DATA OUT CE, UPDATE CE, UPDATE DATA OUT DATA OUT

2.0 V min 0.8 V max 2.7 V min 0.5 V max 20 µA max –400 µA min –400 µA max 3.0 mA min

V

IL

V

OH

V

OL

I

IH

I

IL

I

OH

I

OL

–3–REV. A

AD8108/AD8109

TIMING CHARACTERISTICS (Parallel)

Limit

Parameter Symbol Min Max Unit

Data Setup Time t
CLK Pulsewidth t
Data Hold Time t
CLK Pulse Separation t
CLK to UPDATE Delay t
UPDATE Pulsewidth t

1

2

3

4

5

6

20 ns
100 ns
20 ns
100 ns
0ns
50 ns

Propagation Delay, UPDATE to Switch On or Off – 8 ns
CLK, UPDATE Rise and Fall Times – 100 ns
RESET Time – 200 ns

CLK

D0–D3
A0–A2

1 = LATCHED

UPDATE

0 = TRANSPARENT

t

1

0

t

1

1

0

2

t

3

t

4

t6t

5

Figure 2. Timing Diagram, Parallel Mode

Table II. Logic Levels

V

IH

V

IL

V

OH

V

OL

I

IH

I

IL

I

OH

I

OL

RESET, SER/PAR RESET, SER/PAR RESET, SER/PAR RESET, SER/PAR
CLK, D0, D1, D2, CLK, D0, D1, D2, CLK, D0, D1, D2, CLK, D0, D1, D2,
D3, A0, A1, A2 D3, A0, A1, A2 D3, A0, A1, A2 D3, A0, A1, A2
CE, UPDATE CE, UPDATE DATA OUT DATA OUT CE, UPDATE CE, UPDATE DATA OUT DATA OUT

2.0 V min 0.8 V max 2.7 V min 0.5 V max 20 µA max –400 µA min –400 µA max 3.0 mA min

–4–

REV. A

AD8108/AD8109

AMBIENT TEMPERATURE – C

5.0

MAXIMUM POWER DISSIPATION – W

4.0

0

–50 80–40 –30 –20 –100 10203040506070

3.0

2.0

1.0

TJ = 150C

90

ABSOLUTE MAXIMUM RATINGS

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.0 V

Internal Power Dissipation

2

1

AD8108/AD8109 80-Lead Plastic LQFP (ST) . . . . . 2.6 W

Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±V

S

Output Short Circuit Duration

. . . . . . . . . . . . . . . . . . . . . . . Observe Power Derating Curves

Storage Temperature Range . . . . . . . . . . . . –65°C to +125°C

Lead Temperature Range (Soldering 10 sec) . . . . . . . . . 300°C

NOTES

1

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.

2

Specification is for device in free air (TA = 25°C):

80-lead plastic LQFP (ST): θJA = 48°C/W.

Model Range Description Option

AD8108AST –40°C to +85°C 80-Lead Plastic LQFP ST-80A

AD8109AST –40°C to +85°C 80-Lead Plastic LQFP ST-80A

AD8108-EB Evaluation Board
AD8109-EB Evaluation Board

ORDERING GUIDE

Temperature Package Package

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD8108/AD8109 features proprietary ESD protection circuitry, permanent dam­age may occur on devices subjected to high-energy electrostatic discharges. Therefore, proper
ESD precautions are recommended to avoid performance degradation or loss of functionality.

(12 mm × 12 mm)

(12 mm × 12 mm)

WARNING!

ESD SENSITIVE DEVICE

MAXIMUM POWER DISSIPATION

The maximum power that can be safely dissipated by the
AD8108/AD8109 is limited by the associated rise in junction
temperature. The maximum safe junction temperature for plastic
encapsulated devices is determined by the glass transition tem­perature of the plastic, approximately 150°C. Temporarily
exceeding this limit may cause a shift in parametric performance
due to a change in the stresses exerted on the die by the package.
Exceeding a junction temperature of 175°C for an extended
period can result in device failure.

While the AD8108/AD8109 is internally short circuit protected,
this may not be sufficient to guarantee that the maximum junction
temperature (150°C) is not exceeded under all conditions. To
ensure proper operation, it is necessary to observe the maximum
power derating curves shown in Figure 3.

Figure 3. Maximum Power Dissipation vs. Temperature

–5–REV. A

AD8108/AD8109

PIN CONFIGURATION

1

IN00

2

AGND

3

IN01

4

AGND

5

IN02

6

AGND

7

IN03

8

AGND

9

IN04

10

AGND

11

IN05

12

AGND

13

IN06

14

AGND

15

IN07

16

AGND

17

AVEE

18

AVCC

19

AVCC07

20

OUT07

NC = NO CONNECT

NC

NC

NC

NC

NC

DVCC

DGND

80

79787776757473727170696867

PIN 1
IDENTIFIER

NC

NC

NC

NC

AD8108/AD8109

TOP VIEW

(Not to Scale)

22

23

21

AGND07

AVEE06/07

24

OUT06

AGND06

25

26

OUT05

AVCC05/06

27

28

AGND05

AVEE04/05

30

29

OUT04

AGND04

31

AVCC03/04

NC

NC

33

32

OUT03

AGND03

NC

NC

NC

66

656463

35

36

34

OUT02

AGND02

AVEE02/03

DGND

DVCC

NC

62

37

38

39

OUT01

AGND01

AVCC01/02

RESET

61

60

59

58

57

56

55

54

53

52

51

50

49

48

47

46

45

44

43

42

41

40

AVEE00/01

CE

DATA OUT

CLK

DATA IN

UPDATE
SER/PAR

A0

A1

A2

D0

D1

D2

D3

NC

AGND

AVEE

AVCC

AVCC00

AGND00

OUT00

–6–

REV. A

AD8108/AD8109

PIN FUNCTION DESCRIPTIONS

Pin Name Pin Numbers Pin Description

INxx 1, 3, 5, 7, 9, 11, 13, 15 Analog Inputs; xx = Channel Numbers 00 Through 07.
DATA IN 57 Serial Data Input, TTL Compatible.
CLK 58 Clock, TTL Compatible. Falling Edge Triggered.
DATA OUT 59 Serial Data Output, TTL Compatible.
UPDATE 56 Enable (Transparent) “Low.” Allows serial register to connect directly to switch

matrix. Data latched when “High.”

RESET 61 Disable Outputs, Active “Low.”
CE 60 Chip Enable, Enable “Low.” Must be “low” to clock in and latch data.
SER/PAR 55 Selects Serial Data Mode, “Low” or Parallel Data Mode, “High.” Must be connected.

OUTyy 41, 38, 35, 32, 29, 26, 23, 20 Analog Outputs yy = Channel Numbers 00 Through 07.
AGND 2, 4, 6, 8, 10, 12, 14, 16, 46 Analog Ground for Inputs and Switch Matrix.
DVCC 63, 79 5 V for Digital Circuitry
DGND 62, 80 Ground for Digital Circuitry
AVEE 17, 45 –5 V for Inputs and Switch Matrix.
AVCC 18, 44 +5 V for Inputs and Switch Matrix.
AGNDxx 42, 39, 36, 33, 30, 27, 24, 21 Ground for Output Amp, xx = Output Channel Numbers 00 Through 07. Must be connected.
AVCCxx/yy 43, 37, 31, 25, 22, 19 +5 V for Output Amplifier that is shared by Channel Numbers xx and yy. Must be connected.
AVEExx/yy 40, 34, 28, 22 –5 V for Output Amplifier that is shared by Channel Numbers xx and yy. Must be connected.
A0 54 Parallel Data Input, TTL Compatible (Output Select LSB).
A1 53 Parallel Data Input, TTL Compatible (Output Select).
A2 52 Parallel Data Input, TTL Compatible (Output Select MSB).
D0 51 Parallel Data Input, TTL Compatible (Input Select LSB).
D1 50 Parallel Data Input, TTL Compatible (Input Select).
D2 49 Parallel Data Input, TTL Compatible (Input Select MSB).
D3 48 Parallel Data Input, TTL Compatible (Output Enable).
NC 47, 64–78 No Connect.

V

CC

AVEE

INPUT

ESD

ESD

V

CC

AVEE

a. Analog Input c. Reset Inputb. Analog Output

V

CC

ESD

INPUT

ESD

DGND

d. Logic Input e. Logic Output

Figure 5. I/O Schematics

ESD

ESD

OUTPUT

1k
(AD8109 ONLY)

2k

V

CC

DGND

RESET

ESD

ESD

V

CC

ESD

ESD

OUTPUT

20k

DGND

–7–REV. A

AD8108/AD8109

Table III. Operation Truth Table

SER/

CE UPDATE CLK DATA IN DATA OUT RESET PAR Operation/Comment

1 X X X X X X No change in logic.
01 f Data

i

01 f D0 . . . D3, NA in Parallel 1 1 The data on the parallel data lines, D0–D3, are

A0...A2 Mode loaded into the 32-bit serial shift register loca-

0 0 X X X 1 X Data in the 32-bit shift register transfers into the

X X X X X 0 X Asynchronous operation. All outputs are disabled.

D0

PARALLEL DATA

(OUTPUT ENABLE)

SER/PAR

DATA IN

(SERIAL)

D1
D2
D3

S

D1

Q

D0

D

CLK

S

D1

Q

DQ

Q

D0

CLK

D1

D0

S

Q

Data

DQ

CLK

i-32

1 0 The data on the serial DATA IN line is loaded

into serial register. The first bit clocked into
the serial register appears at DATA OUT 32
clocks later.

tion addressed by A0–A2.

parallel latches that control the switch array.
Latches are transparent.

Remainder of logic is unchanged.

S

D1

Q

D0

DQ

CLK

S

D1

DQ

Q

D0

CLK

S

D1

Q

D0

DQ

CLK

S

D1

Q

D0

DQ

CLK

S

D1

Q

D0

DQ

CLK

S

D1

Q

D0

DQ

CLK

S

D1

Q

Q

D

CLK

DATA
OUT

D0

CLK

CE

UPDATE

OUT0 EN

OUT1 EN

OUT2 EN

A0

OUT3 EN

A1

OUT4 EN

A2

OUT5 EN

3 TO 8 DECODER

OUT6 EN

OUT7 EN

(OUTPUT ENABLE)

RESET

LE

OUT0

LE

OUT0

B1

D

Q

D

B0

Q

LE

OUT0

B2

D

Q

LE

OUT0

EN

D

QCLR

LE

OUT1

B0

D

Q

LE

OUT6

EN

D

QCLR

LE

OUT7

B0

D

Q

LE

OUT7

D

B1

Q

LE

OUT7

D

B2

Q

LE

OUT7

EN

D

QCLR

DECODE

64

8

OUTPUT ENABLESWITCH MATRIX

Figure 4. Logic Diagram

–8–

REV. A

+1.0V

+0.5V

0

–0.5V

–1.0V

500mV/DIV

10ns/DIV

Typical Performance Characteristics—

AD8108/AD8109

5

RL = 150

4

3

2

1

GAIN – dB

0

–1

–2

–3

100k 1M 1G

FLATNESS

GAIN

2V p-p

10M 100M

FREQUENCY – Hz

200mV p-p

TPC 1. AD8108 Frequency Response

–10

RL = 1k

–20

–30

–40

–50

–60

–70

CROSSTALK – dB

–80

–90

–100

–110

0.2 1 20010 100

ALL HOSTILE

ADJACENT

FREQUENCY – MHz

TPC 2. AD8108 Crosstalk vs. Frequency

0.4

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

FLATNESS – dB

+50mV

+25mV

0

25mV/DIV

–25mV

–50mV

10ns/DIV

TPC 4. AD8108 Step Response, 100 mV Step

TPC 5. AD8108 Step Response, 2 V Step

–30

RL = 150
V

= 2V p-p

OUT

–40

–50

–60

–70

DISTORTION – dB

–80

–90

–100

100k 1M 10M 100M

2ND HARMONIC

3RD HARMONIC

FREQUENCY – Hz

TPC 3. AD8108 Distortion vs. Frequency

2V STEP

= 150

R

L

0.2

0.1

0

0.1%/DIV

–0.1

–0.2

0 10203040 50607080

10ns/DIV

TPC 6. AD8108 Settling Time

–9–REV. A

AD8108/AD8109

5

4

3

2

1

GAIN – dB

0

–1

–2

–3

100k 1M 1G10M 100M

FLATNESS

GAIN

2V p-p

2V p-p

FREQUENCY – Hz

TPC 7. AD8109 Frequency Response

–20

RL = 1k

–30

–40

–50

–60

–70

–80

CROSSTALK – dB

–90

–100

–110

300k 1M 10M 100M

ADJACENT

ALL HOSTILE

FREQUENCY – Hz

TPC 8. AD8109 Crosstalk vs. Frequency

200mV p-p

0.4

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

200M

FLATNESS – dB

+50mV

+25mV

0

25mV/DIV

–25mV

–50mV

10ns/DIV

TPC 10. AD8109 Step Response, 100 mV Step

+1.0V

+0.5V

0

0.5V/DIV

–0.5V

–1.0V

10ns/DIV

TPC 11. AD8109 Step Response, 2 V Step

–30

RL = 150

–40

V

= 2V p-p

OUT

–50

–60

–70

DISTORTION – dB

–80

–90

–100

100k 1M 10M 100M

2ND HARMONIC

3RD HARMONIC

FREQUENCY – Hz

TPC 9. AD8109 Distortion vs. Frequency

–10–

0.2

0.1

0

0.1%/DIV

–0.1

–0.2

2V STEP

= 150

R

L

0 1020 3040 50 60 7080

10ns/DIV

TPC 12. AD8109 Settling Time

REV. A

–30

OFF ISOLATION – dB

FREQUENCY – Hz

100k 10M 100M 500M

1M

VIN = 2V p-p
R

L

= 150

–50

–60

–70

–80

–90

–100

–110

–120

–130

–40

–140

OUTPUT IMPEDANCE – 

1k

100

10

1

FREQUENCY – Hz

100k 10M 100M 500M

0.1
1M

RL = 150

–40

–50

–60

–70

–80

POWER SUPPLY REJECTION – dB

–90

10k 100k 1M 10M

FREQUENCY – Hz

TPC 13. AD8108 PSRR vs. Frequency

100

56.3

31.6

AD8108/AD8109

5

4

3

1V/DIV10mV/DIV

2

1

0

10

0

–10

TPC 16. AD8108 Switching Transient (Glitch)

SWITCHING BETWEEN

TWO INPUTS

UPDATE INPUT

TYPICAL VIDEO OUT (RTO)

50ns/DIV

17.8

nV/ Hz

10

5.63

3.16

FREQUENCY – Hz

TPC 14. AD8108 Voltage Noise vs. Frequency

1M

100k

10k

OUTPUT IMPEDANCE – 

1k

100

0.1 10 100 500

1

FREQUENCY – MHz

TPC 15. AD8108 Output Impedance, Disabled

10k1k100

100k 1M 10M10

TPC 17. AD8108 Off Isolation, Input-Output

TPC 18. AD8108 Output Impedance, Enabled

–11–REV. A

AD8108/AD8109

–30

RL = 150

–40

–50

–60

–70

–80

POWER SUPPLY REJECTION – dB RTI

–90

10k 100k 1M 10M

TPC 19. AD8109 PSRR vs. Frequency

FREQUENCY – Hz

5

4

3

1V/DIV10mV/DIV

2

1

0

10

0

–10

SWITCHING BETWEEN

TWO INPUTS

UPDATE INPUT

TYPICAL VIDEO OUT (RTO)

50ns/DIV

TPC 22. AD8109 Switching Transient (Glitch)

100.0

56.3

31.6

17.8

nV/ Hz

10.0

5.63

3.16

FREQUENCY – Hz

100k 1M 10M10

10k1k100

TPC 20. AD8109 Voltage Noise vs. Frequency

100k

10k

1k

–40

V

= 2V p-p

OUT

–50

R

= 150

L

–60

–70

–80

–90

–100

–110

OFF ISOLATION – dB

–120

–130

–140

100k 10M 100M 500M

1M

FREQUENCY – Hz

TPC 23. AD8109 Off Isolation, Input-Output

1k

100

10

OUTPUT IMPEDANCE – 

100

1
100k 10M 100M 500M

1M

FREQUENCY – Hz

TPC 21. AD8109 Output Impedance, Disabled

–12–

OUTPUT IMPEDANCE – 

1

0.1
100k 10M 100M 500M

1M

FREQUENCY – Hz

TPC 24. AD8109 Output Impedance, Enabled

REV. A

100k

TEMPERATURE – C

V

OS

– mV

–60

2.0

1.5

0.0

–1.0

–2.0

1.0

0.5

–0.5

–1.5

–40 –20 0 20 40 60 80 100

INPUT IMPEDANCE – 

1M

10k

AD8108/AD8109

V

1

0

1V/DIV2V/DIV

–1

5

1k

0

INPUT 1 AT +1V

INPUT 0 AT –1V

OUT

UPDATE

100

30k

1M 500M10M 100M100k

FREQUENCY – Hz

TPC 25. AD8108 Input Impedance vs. Frequency

VIN = 200mV

8

R

= 150

GAIN – dB

L

6

4

2

0

–2

–4

–6

–8

FREQUENCY – Hz

CL = 18pF

CL = 12pF

100M1M 10M30k 3G1G100k

TPC 26. AD8108 Frequency Response vs. Capacitive Load

0.5

VIN = 200mV

0.4
R

= 150

FLATNESS – dB

L

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

–0.5

CL = 18pF

CL = 12pF

FREQUENCY – Hz

100M1M 10M30k 3G1G100k

TPC 27. AD8108 Flatness vs. Capacitive Load

50ns/DIV

TPC 28. AD8108 Switching Time

900

800

700

600

500

400

FREQUENCY

300

200

100

0

–0.020

–0.010 0.000 0.010 0.020

OFFSET VOLTAGE – Volts

TPC 29. AD8108 Offset Voltage Distribution

TPC 30. AD8108 Offset Voltage Drift vs. Temperature

°

(Normalized at 25

C)

–13–REV. A

AD8108/AD8109

1M

100k

10k

INPUT IMPEDANCE – 

1k

1

0

1V/DIV2V/DIV

–1

5

0

INPUT 1 AT +1V

INPUT 0 AT –1V

V

OUT

UPDATE

100

30k

1M 500M10M 100M100k

FREQUENCY – Hz

TPC 31. AD8109 Input Impedance vs. Frequency

VIN = 100mV

8

R

= 150

L

GAIN – dB

6

4

2

0

–2

–4

–6

–8

FREQUENCY – Hz

CL = 18pF

CL = 12pF

100M1M 10M30k 3G1G100k

TPC 32. AD8109 Frequency Response vs. Capacitive
Load

VIN = 100mV

0.4
R

= 150

L

GAIN – dB

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

CL = 18pF

CL = 12pF

FREQUENCY – Hz

100M1M 10M30k 3G1G100k

TPC 33. AD8109 Flatness vs. Capacitive Load

50ns/DIV

TPC 34. AD8109 Switching Time

320

300

280

260

240

220

200

180

160

140

FREQUENCY

120

100

80

60

40

0

–0.020

–0.010 0.000 0.010 0.020

OFFSET VOLTAGE – V

TPC 35. AD8109 Offset Voltage Distribution (RTI)

2.0

1.5

1.0

0.5

– mV

0.0

OS

V

–0.5

–1.0

–1.5

–2.0

–60

–40 –200 20406080100

TEMPERATURE – C

TPC 36. AD8109 Offset Voltage Drift vs. Temperature

°

(Normalized at 25

C)

–14–

REV. A

AD8108/AD8109

THEORY OF OPERATION

The AD8108 (G = +1) and AD8109 (G = +2) share a common
core architecture consisting of an array of 64 transconductance
(gm) input stages organized as eight 8:1 multiplexers with a
common, 8-line analog input bus. Each multiplexer is basically a
folded-cascode high-impedance voltage feedback amplifier with
eight input stages. The input stages are NPN differential pairs
whose differential current outputs are combined at the output
stage, which contains the high impedance node, compensation
and a complementary emitter follower output buffer. In the
AD8108, the output of each multiplexer is fed back directly to the
inverting inputs of its eight gm stages. In the AD8109, the
feedback network is a voltage divider consisting of two equal
resistors.

This switched-gm architecture results in a low power crosspoint
switch that is able to directly drive a back terminated video load
(150 Ω) with low distortion (differential gain and differential
phase errors are better than 0.02% and 0.02°, respectively).
This design also achieves high input resistance and low input
capacitance without the signal degradation and power dissipation
of additional input buffers. However, the small input bias current
at any input will increase almost linearly with the number of
outputs programmed to that input.

The output disable feature of these crosspoints allows larger
switch matrices to be built by simply busing together the outputs
of multiple 8 × 8 ICs. However, while the disabled output imped­ance of the AD8108 is very high (10 MΩ), that of the AD8109
is limited by the resistive feedback network (which has a nominal
total resistance of 1 kΩ that appears in parallel with the disabled
output. If the outputs of multiple AD8109s are connected through
separate back termination resistors, the loading due to these finite
output impedances will lower the effective back termination
impedance of the overall matrix. This problem is eliminated if the
outputs of multiple AD8109s are connected directly and share a
single back termination resistor for each output of the overall
matrix. This configuration increases the capacitive loading of the
disabled AD8109s on the output of the enabled AD8109.

APPLICATIONS

The AD8108/AD8109 have two options for changing the pro­gramming of the crosspoint matrix. In the first, a serial word of 32
bits can be provided that will update the entire matrix each time.
The second option allows for changing a single output’s program­ming via a parallel interface. The serial option requires fewer
signals, but requires more time (clock cycles) for changing the
programming, while the parallel programming technique requires
more signals, but can change a single output at a time and requires
fewer clock cycles to complete programming.

Serial Programming

The serial programming mode uses the device pins CE, CLK,
DATA IN, UPDATE, and SER/PAR. The first step is to assert
a LOW on SER/PAR in order to enable the serial programming
mode. CE for the chip must be LOW to allow data to be clocked
into the device. The CE signal can be used to address an individual
device when devices are connected in parallel.

The UPDATE signal should be HIGH during the time that data
is shifted into the device’s serial port. Although the data will still
shift in when UPDATE is LOW, the transparent, asynchronous
latches will allow the shifting data to reach the matrix. This will
cause the matrix to try to update to every intermediate state as
defined by the shifting data.

The data at DATA IN is clocked in at every down edge of CLK.
A total of 32 data bits must be shifted in to complete the program­ming. For each of the eight outputs, there are three bits (D0–D2)
that determine the source of its input followed by one bit (D3)
that determines the enabled state of the output. If D3 is LOW
(output disabled), the three associated bits (D0–D2) do not
matter because no input will be switched to that output.

The most significant output address data is shifted in first, then
following in sequence until the least significant output address
data is shifted in. At this point UPDATE can be taken LOW,
which will cause the programming of the device according to the
data that was just shifted in. The UPDATE registers are asyn­chronous and when UPDATE is LOW, they are transparent.

If more than one AD8108/AD8109 device is to be serially pro­grammed in a system, the DATA OUT signal from one device
can be connected to the DATA IN of the next device to form a
serial chain. All of the CLK, CE, UPDATE, and SER/PAR pins
should be connected in parallel and operated as described
above. The serial data is input to the DATA IN pin of the first
device of the chain, and it will ripple on through to the last.
Therefore, the data for the last device in the chain should come
at the beginning of the programming sequence. The length of the
programming sequence will be 32 times the number of devices
in the chain.

PARALLEL PROGRAMMING

When using the parallel programming mode, it is not necessary
to reprogram the entire device when making changes to the
matrix. In fact, parallel programming allows the modification of
a single output at a time. Since this takes only one CLK/UPDATE
cycle, significant time savings can be realized by using parallel
programming.

One important consideration in using parallel programming is that
the RESET signal does not reset all registers in the AD8108/AD8109.
When taken low, the RESET signal will only set each output to the
disabled state. This is helpful during power-up to ensure that two
parallel outputs will not be active at the same time.

After initial power-up, the internal registers in the device will
generally have random data, even though the RESET signal was
asserted. If parallel programming is used to program one output,
that output will be properly programmed but the rest of the device
will have a random program state depending on the internal
register content at power-up. Therefore, when using parallel
programming, it is essential that all outputs be programmed to a
desired state after power-up. This will ensure that the programming
matrix is always in a known state. From then on, parallel program­ming can be used to modify a single, or more, output at a time.

–15–REV. A

AD8108/AD8109

In a similar fashion, if both CE and UPDATE are taken LOW after
initial power-up, the random power-up data in the shift register will
be programmed into the matrix. Therefore, in order to prevent the
crosspoint from being programmed into an unknown state do

not apply low logic levels to both CE and
initially applied. Programming the full shift register one time to a

desired state by either serial or parallel programming after initial
power-up will eliminate the possibility of programming the
matrix to an unknown state.

To change an output’s programming via parallel programming,
SER/PAR and UPDATE should be taken HIGH and CE should
be taken LOW. The CLK signal should be in the HIGH state.
The address of the output that is to be programmed should be
put on A0–A2. The first three data bits (D0–D2) should contain
the information that identifies the input that is programmed to the
output that is addressed. The fourth data bit (D3) will determine
the enabled state of the output. If D3 is LOW (output disabled)
the data on D0–D2 does not matter.

After the desired address and data signals have been established,
they can be latched into the shift register by a HIGH to LOW
transition of the CLK signal. The matrix will not be programmed,
however, until the UPDATE signal is taken low. Thus, it is pos­sible to latch in new data for several or all of the outputs first via
successive negative transitions of CLK while UPDATE is held
high, and then have all the new data take effect when UPDATE
goes LOW. This is the technique that should be used when pro­gramming the device for the first time after power-up when
using parallel programming.

POWER-ON RESET

When powering up the AD8108/AD8109 it is usually desirable
to have the outputs come up in the disabled state. The RESET
pin, when taken LOW will cause all outputs to be in the dis­abled state. However, the RESET signal does not reset all registers
in the AD8108/AD8109. This is important when operating in
the parallel programming mode. Please refer to that section for
information about programming internal registers after power­up. Serial programming will program the entire matrix each
time, so no special considerations apply.

Since the data in the shift register is random after power-up,
they should not be used to program the matrix or else the matrix
can enter unknown states. To prevent this, do not apply logic low

CE

and

signals to both

register should first be loaded with the desired data, and then
UPDATE can be taken LOW to program the device.

The RESET pin has a 20 kΩ pull-up resistor to DVDD that can
be used to create a simple power-up reset circuit. A capacitor
from RESET to ground will hold RESET LOW for some time
while the rest of the device stabilizes. The LOW condition will
cause all the outputs to be disabled. The capacitor will then
charge through the pull-up resistor to the HIGH state, thus
allowing full programming capability of the device.

UPDATE

UPDATE

initially after power-up. The shift

after power is

Gain Selection

The 8 × 8 crosspoints come in two versions depending on the
desired gain of the analog circuit paths. The AD8108 device is
unity gain and can be used for analog logic switching and other
applications where unity gain is desired. The AD8108 can also
be used for the input and interior sections of larger crosspoint
arrays where termination of output signals is not usually used.
The AD8108 outputs have a very high impedance when their
outputs are disabled.

For devices that will be used to drive a terminated cable with its
outputs, the AD8109 can be used. This device has a built-in gain
of two that eliminates the need for a gain-of-two buffer to drive
a video line. Because of the presence of the feedback network in
these devices, the disabled output impedance is about 1 kΩ.

If external amplifiers will be used to provide a G = +2, Analog
Devices’ AD8079 is a fixed gain of +2 buffer.

Creating Larger Crosspoint Arrays

The AD8108/AD8109 are high density building blocks for cre­ating crosspoint arrays of dimensions larger than 8 × 8. Various
features such as output disable, chip enable, and gain-of-one
and-two options are useful for creating larger arrays. For very
large arrays, they can be used along with the AD8116, a 16 × 16
video crosspoint device. In addition, systems that require more
inputs than outputs can use the AD8110 and/or the AD8111,
which are (gain-of-one and gain-of-two) 16 × 8 crosspoint
switches.

The first consideration in constructing a larger crosspoint is to
determine the minimum number of devices required. The 8 × 8
architecture of the AD8108/AD8109 contains 64 “points,” which
is a factor of 16 greater than a 4 × 1 crosspoint. The PC board
area and power consumption savings are readily apparent when
compared to using these smaller devices.

For a nonblocking crosspoint, the number of points required is
the product of the number of inputs multiplied by the number
of outputs. Nonblocking requires that the programming of a given
input to one or more outputs does not restrict the availability of
that input to be a source for any other outputs.

Some nonblocking crosspoint architectures will require more than
this minimum as calculated above. Also, there are blocking
architectures that can be constructed with fewer devices than this
minimum. These systems have connectivity available on a statisti­cal basis that is determined when designing the overall system.

The basic concept in constructing larger crosspoint arrays is to
connect inputs in parallel in a horizontal direction and to “wire-OR”
the outputs together in the vertical direction. The meaning of
horizontal and vertical can best be understood by looking at a diagram.

An 8 input by 16 output crosspoint array can be constructed as
shown in Figure 6. This configuration parallels two inputs per
channel and does not require paralleling of any outputs. Inputs are
easier to parallel than outputs, because there are lower parasitics
involved. For a 16 × 8 crosspoint, the AD8110 (gain of one) or
AD8111 (gain of two) device can be used. These devices are already
configured into a 16 × 8 crosspoint in a single device.

–16–

REV. A

AD8108/AD8109

AD8108

8

ONE

PER INPUT

8

8

OR

AD8109

AD8108

OR

AD8109

8

16 OUTPUTS

OUT 00–15

8

8 INPUTS

IN 00–07

TERMINATION

Figure 6. 8 × 16 Crosspoint Array Using Two AD8108s
(Unity Gain) or Two AD8109s (Gain-of-Two)

Figure 7 illustrates a 16 × 16 crosspoint array, while a 24 × 24
crosspoint is illustrated in Figure 8. The 16 × 16 crosspoint
requires that each input driver drive two inputs in parallel and
each output be wire-ORed with one other output. The 24 × 24
crosspoint requires driving three inputs in parallel and having
the outputs wire-ORed in groups of three. It is required of the
system programming that only one output of a wired-OR node
be active at a time.

IN 00–07

8

8  8 8  8

8

00–07

8

R

TERM

8

At some point, the number of outputs that are wire-ORed becomes
too great to maintain system performance. This will vary according
to which system specifications are most important. For example,
a 64 × 8 crosspoint can be created with eight AD8108/AD8109s.
This design will have 64 separate inputs and have the correspond­ing outputs of each device wire-ORed together in groups of eight.

Using additional crosspoint devices in the design can lower the
number of outputs that must be wire-ORed together. Figure 9
shows a block diagram of a system using eight AD8108s and
two AD8109s to create a nonblocking, gain-of-two, 64 × 8 cross­point that restricts the wire-ORing at the output to only four
outputs. The rank 1 wire-ORed devices are the AD8108,
which has a higher disabled output impedance than the AD8109.

RANK 1

(64:16)

IN 00–07

IN 08–15

IN 16–23

IN 24–31

IN 32–39

IN 40–47

8

8

8

8

8

8

AD8108

AD8108

AD8108

AD8108

AD8108

AD8108

4

4

4

4

4

4

4

4

4

4

4

4

4

1k

4

1k

RANK 2

16  8 NONBLOCKING

16  16 BLOCKING

4

1k

4

1k

AD8109

AD8109

4

4

4

4

OUT 00–07
NONBLOCKING

ADDITIONAL
8 OUTPUTS
(SUBJECT TO
BLOCKING)

IN 08–15

8

8  8 8  8

8

OUT 00–07

08–15

8

R

TERM

8

OUT 08–15

Figure 7. 16 × 16 Crosspoint Array Using Four AD8108s
or AD8109s

IN 00–07

IN 08–15

IN 16–23

8

8  8

8 8

8

8  8 8  8 8  8

8 8

8

8  8 8  8 8  8

8 8 8

OUT 00–07

8  8 8  8

OUT 08–15

8

8

R

8

R

R

TERM

TERM

TERM

8

8

OUT 16–23

Figure 8. 24 × 24 Crosspoint Array Using Nine AD8108s or
AD8109s

IN 48–55

IN 56–63

8

8

AD8108

AD8108

4

4

4

4

Figure 9. Nonblocking 64 × 8 Array with Gain-of-Two

×

16 Blocking)

(64

Additionally, by using the lower four outputs from each of the
two rank 2 AD8109s, a blocking 64 × 16 crosspoint array can be
realized. There are, however, some drawbacks to this technique.
The offset voltages of the various cascaded devices will accumu­late and the bandwidth limitations of the devices will compound.
In addition, the extra devices will consume more current and
take up more board space. Once again, the overall system design
specifications will determine how to make the various tradeoffs.

Multichannel Video

The excellent video specifications of the AD8108/AD8109 make
them ideal candidates for creating composite video crosspoint
switches. These can be made quite dense by taking advantage
of the AD8108/AD8109’s high level of integration and the fact
that composite video requires only one crosspoint channel per
system video channel. There are, however, other video formats
that can be routed with the AD8108/AD8109 requiring more
than one crosspoint channel per video channel.

–17–REV. A

AD8108/AD8109

Some systems use twisted-pair wiring to carry video signals.
These systems utilize differential signals and can lower costs
because they use lower cost cables, connectors and termination
methods. They also have the ability to lower crosstalk and reject
common-mode signals, which can be important for equipment
that operates in noisy environments or where common-mode volt­ages are present between transmitting and receiving equipment.

In such systems, the video signals are differential; there is a positive
and negative (or inverted) version of the signals. These comple­mentary signals are transmitted onto each of the two wires of the
twisted pair, yielding a first order zero common- mode signal. At
the receive end, the signals are differentially received and converted
back into a single-ended signal.

When switching these differential signals, two channels are
required in the switching element to handle the two differential
signals that make up the video channel. Thus, one differential video
channel is assigned to a pair of crosspoint channels, both input
and output. For a single AD8108/AD8109, four differential video
channels can be assigned to the eight inputs and eight outputs.
This will effectively form a 4 × 4 differential crosspoint switch.

Programming such a device will require that inputs and outputs
be programmed in pairs. This information can be deduced by
inspection of the programming format of the AD8108/AD8109
and the requirements of the system.

There are other analog video formats requiring more than one
analog circuit per video channel. One two-circuit format that is
commonly being used in systems such as satellite TV, digital cable
boxes and higher quality VCRs, is called S-video or Y/C video.
This format carries the brightness (luminance or Y) portion of the
video signal on one channel and the color (chrominance, chroma
or C) on a second channel.

Since S-video also uses two separate circuits for one video chan­nel, creating a crosspoint system requires assigning one video
channel to two crosspoint channels as in the case of a differen­tial video system. Aside from the nature of the video format,
other aspects of these two systems will be the same.

There are yet other video formats using three channels to carry
the video information. Video cameras produce RGB (red, green,
blue) directly from the image sensors. RGB is also the usual
format used by computers internally for graphics. RGB can also
be converted to Y, R–Y, B–Y format, sometimes called YUV
format. These three-circuit, video standards are referred to as
component analog video.

The component video standards require three crosspoint chan­nels per video channel to handle the switching function. In a
fashion similar to the two-circuit video formats, the inputs and
outputs are assigned in groups of three and the appropriate logic
programming is performed to route the video signals.

CROSSTALK

Many systems, such as broadcast video, that handle numerous
analog signal channels have strict requirements for keeping the
various signals from influencing any of the others in the system.
Crosstalk is the term used to describe the coupling of the signals
of other nearby channels to a given channel.

When there are many signals in proximity in a system, as will
undoubtedly be the case in a system that uses the AD8108/
AD8109, the crosstalk issues can be quite complex. A good
understanding of the nature of crosstalk and some definition of

terms is required in order to specify a system that uses one or
more AD8108/AD8109s.

TYPES OF CROSSTALK

Crosstalk can be propagated by means of any of three methods.
These fall into the categories of electric field, magnetic field and
sharing of common impedances. This section will explain these effects.

Every conductor can be both a radiator of electric fields and a
receiver of electric fields. The electric field crosstalk mechanism
occurs when the electric field created by the transmitter propagates
across a stray capacitance (e.g., free space) and couples with the
receiver and induces a voltage. This voltage is an unwanted crosstalk
signal in any channel that receives it.

Currents flowing in conductors create magnetic fields that circulate
around the currents. These magnetic fields will then generate
voltages in any other conductors whose paths they link. The undes­ired induced voltages in these other channels are crosstalk signals.
The channels that crosstalk can be said to have a mutual induc­tance that couples signals from one channel to another.

The power supplies, grounds and other signal return paths of a multi­channel system are generally shared by the various channels. When
a current from one channel flows in one of these paths, a voltage
that is developed across the impedance becomes an input crosstalk
signal for other channels that share the common impedance.

All these sources of crosstalk are vector quantities, so the mag­nitudes cannot simply be added together to obtain the total
crosstalk. In fact, there are conditions where driving additional
circuits in parallel in a given configuration can actually reduce the
crosstalk.

Areas of Crosstalk

For a practical AD8108/AD8109 circuit, it is required that it be
mounted to some sort of circuit board in order to connect it to
power supplies and measurement equipment. Great care has been
taken to create a characterization board (also available as an evalu­ation board) that adds minimum crosstalk to the intrinsic device.
This, however, raises the issue that a system’s crosstalk is a combi­nation of the intrinsic crosstalk of the devices in addition to the
circuit board to which they are mounted. It is important to try to
separate these two areas of crosstalk when attempting to minimize
its effect.

In addition, crosstalk can occur among the inputs to a crosspoint
and among the outputs. It can also occur from input to output.
Techniques will be discussed for diagnosing which part of a system
is contributing to crosstalk.

Measuring Crosstalk

Crosstalk is measured by applying a signal to one or more chan­nels and measuring the relative strength of that signal on a desired
selected channel. The measurement is usually expressed as dB
down from the magnitude of the test signal. The crosstalk is
expressed by:

|XT| = 20 log

where s = jω is the Laplace transform variable, Asel(s) is the
amplitude of the crosstalk-induced signal in the selected channel
and Atest(s) is the amplitude of the test signal. It can be seen
that crosstalk is a function of frequency, but not a function of
the magnitude of the test signal (to first order). In addition, the
crosstalk signal will have a phase relative to the test signal asso­ciated with it.

(Asel(s)/Atest(s))

10

–18–

REV. A

AD8108/AD8109

A network analyzer is most commonly used to measure crosstalk
over a frequency range of interest. It can provide both magnitude
and phase information about the crosstalk signal.

As a crosspoint system or device grows larger, the number of theo­retical crosstalk combinations and permutations can become
extremely large. For example, in the case of the 8 × 8 matrix of the
AD8108/AD8109, we can examine the number of crosstalk
terms that can be considered for a single channel, say IN00
input. IN00 is programmed to connect to one of the AD8108/
AD8109 outputs where the measurement can be made.

We can first measure the crosstalk terms associated with driving
a test signal into each of the other seven inputs one at a time.
We can then measure the crosstalk terms associated with driving
a parallel test signal into all seven other inputs taken two at a time
in all possible combinations; and then three at a time, etc., until,
finally, there is only one way to drive a test signal into all seven
other inputs.

Each of these cases is legitimately different from the others and
might yield a unique value depending on the resolution of the
measurement system, but it is hardly practical to measure all these
terms and then to specify them. In addition, this describes the
crosstalk matrix for just one input channel. A similar crosstalk
matrix can be proposed for every other input. In addition, if the
possible combinations and permutations for connecting inputs
to the other (not used for measurement) outputs are taken into
consideration, the numbers rather quickly grow to astronomical
proportions. If a larger crosspoint array of multiple AD8108/
AD8109s is constructed, the numbers grow larger still.

Obviously, some subset of all these cases must be selected to be
used as a guide for a practical measure of crosstalk. One common
method is to measure “all hostile” crosstalk. This term means that
the crosstalk to the selected channel is measured, while all other
system channels are driven in parallel. In general, this will yield
the worst crosstalk number, but this is not always the case due
to the vector nature of the crosstalk signal.

Other useful crosstalk measurements are those created by one
nearest neighbor or by the two nearest neighbors on either side.
These crosstalk measurements will generally be higher than those
of more distant channels, so they can serve as a worst-case measure
for any other one-channel or two-channel crosstalk measurements.

Input and Output Crosstalk

The flexible programming capability of the AD8108/AD8109
can be used to diagnose whether crosstalk is occurring more on the
input side or the output side. Some examples are illustrative. A
given input channel (IN03 in the middle for this example) can be
programmed to drive OUT03. The input to IN03 is just terminated
to ground (via 50 Ω or 75 Ω) and no signal is applied.

All the other inputs are driven in parallel with the same test signal
(practically provided by a distribution amplifier), with all other
outputs except OUT03 disabled. Since grounded IN03 is pro­grammed to drive OUT03, there should be no signal present.
Any signal that is present can be attributed to the other seven
hostile input signals, because no other outputs are driven. (They
are all disabled.) Thus, this method measures the all-hostile input

contribution to crosstalk into IN03. Of course, the method can be
used for other input channels and combinations of hostile inputs.

For output crosstalk measurement, a single input channel is
driven (IN00, for example) and all outputs other than a given
output (IN03 in the middle) are programmed to connect to
IN00. OUT03 is programmed to connect to IN07 (far away
from IN00), which is terminated to ground. Thus OUT03
should not have a signal present since it is listening to a quiet
input. Any signal measured at the OUT03 can be attributed to
the output crosstalk of the other seven hostile outputs. Again,
this method can be modified to measure other channels and
other crosspoint matrix combinations.

Effect of Impedances on Crosstalk

The input side crosstalk can be influenced by the output imped­ance of the sources that drive the inputs. The lower the
impedance of the drive source, the lower the magnitude of the
crosstalk. The dominant crosstalk mechanism on the input side
is capacitive coupling. The high impedance inputs do not have
significant current flow to create magnetically induced crosstalk.
However, significant current can flow through the input termi­nation resistors and the loops that drive them. Thus, the PC
board on the input side can contribute to magnetically coupled
crosstalk.

From a circuit standpoint, the input crosstalk mechanism looks
like a capacitor coupling to a resistive load. For low frequencies
the magnitude of the crosstalk will be given by:

where R

|XT| = 20 log

is the source resistance, CM is the mutual capacitance

S

[(RS CM) × s]

10

between the test signal circuit and the selected circuit, and s is
the Laplace transform variable.

From the equation it can be observed that this crosstalk mechanism
has a high-pass nature; it can also be minimized by reducing the
coupling capacitance of the input circuits and lowering the output
impedance of the drivers. If the input is driven from a 75 Ω
terminated cable, the input crosstalk can be reduced by buffering
this signal with a low output impedance buffer.

On the output side, the crosstalk can be reduced by driving a
lighter load. Although the AD8108/AD8109 is specified with
excellent differential gain and phase when driving a standard
150 Ω video load, the crosstalk will be higher than the minimum
obtainable due to the high output currents. These currents will
induce crosstalk via the mutual inductance of the output pins
and bond wires of the AD8108/AD8109.

From a circuit standpoint, this output crosstalk mechanism
looks like a transformer, with a mutual inductance between the
windings, that drives a load resistor. For low frequencies, the
magnitude of the crosstalk is given by:

|XT| = 20 log

(Mxy × s/RL)

10

where Mxy is the mutual inductance of output x to output y and

R

is the load resistance on the measured output. This crosstalk

L

mechanism can be minimized by keeping the mutual inductance
low and increasing R

. The mutual inductance can be kept low

L

by increasing the spacing of the conductors and minimizing
their parallel length.

–19–REV. A

AD8108/AD8109

PCB Layout

Extreme care must be exercised to minimize additional crosstalk
generated by the system circuit board(s). The areas that must be
carefully detailed are grounding, shielding, signal routing, and
supply bypassing.

The packaging of the AD8108/AD8109 is designed to help keep
the crosstalk to a minimum. Each input is separated from each
other input by an analog ground pin. All of these AGNDs
should be directly connected to the ground plane of the circuit
board. These ground pins provide shielding, low impedance return
paths and physical separation for the inputs. All of these help to
reduce crosstalk.

Each output is separated from its two neighboring outputs by an
analog ground pin in addition to an analog supply pin of one
polarity or the other. Each of these analog supply pins provides
power to the output stages of only the two nearest outputs. These
supply pins and analog grounds provide shielding, physical sepa­ration and a low impedance supply for the outputs. Individual
bypassing of each of these supply pins, with a 0.01 µF chip capaci-
tor directly to the ground plane, minimizes high frequency output
crosstalk via the mechanism of sharing common impedances.

Each output also has an on-chip compensation capacitor that
is individually tied the nearby analog ground pins AGND00
through AGND07. This technique reduces crosstalk by prevent­ing the currents that flow in these paths from sharing a common
impedance on the IC and in the package pins. These AGNDxx
signals should all be directly connected to the ground plane.

The input and output signals will have minimum crosstalk if they
are located between ground planes on layers above and below, and
separated by ground in between. Vias should be located as close
to the IC as possible to carry the inputs and outputs to the inner
layer. The only place the input and output signals surface is at the
input termination resistors and the output series back termination
resistors. These signals should also be separated, to the extent
possible, as soon as they emerge from the IC package.

Evaluation Board

A four-layer evaluation board for the AD8108/AD8109 is avail­able. The exact same board and external components are used
for each device. The only difference is the device itself, which
offers a selection of a gain of unity or gain of two through the
analog channels. This board has been carefully laid out and tested
to demonstrate the specified high-speed performance of the device.
Figure 10 shows the schematic of the evaluation board. Figure
11 shows the component side silk-screen. The layouts of the
board’s four layers are given in Figures 12, 13, 14, and 15.

The evaluation board package includes the following:

•

Fully populated board with BNC-type connectors.

•

Windows® based software for controlling the board from a PC
via the printer port.

•

Custom cable to connect evaluation board to PC.

•

Disk containing Gerber files of board layout.

Windows is a registered trademark of Microsoft Corporation

–20–

REV. A

DVCC DGND NCAVEE AGND AVCCNC

P1-1

P1-2

+

0.1F10F

P1-3

CR1

CR2

P1-4

1N4148

0.1F10F

INPUT 00 INPUT 00

INPUT 01 INPUT 01

INPUT 02 INPUT 02

INPUT 03 INPUT 03

INPUT 04 INPUT 04

INPUT 05 INPUT 05

INPUT 06 INPUT 06

INPUT 07 INPUT 07

P2-5

P2-4

P2-2

P2-3

P2-1

P2-6

NC = NO CONNECT

P1-5

+

P1-6

0.1F10F

75

75

75

75

75

75

75

75

P1-7

+

80 46

79

1
2

AGND

3
4

AGND

5
6

AGND

7
8

AGND

9

10

AGND

11
12

AGND

13
14

AGND

15
16

AGND

59

DATA OUT

57

DATA IN

DGNDCECLK

62 61 60 58 56 55 54 53 52 51 50 49 48

P3-1

0.01F

RESET

P3-2

P3-3

DVCCDVCC

63

DVCCDVCCDGND

P3-4

AVCC

0.01F

0.01F

43

AVCC

AD8108/AD8109

UPDATE

SER/PARA0A1A2D0D1D2

P3-5

P3-6

P3-7

P3-8

P3-9

AVCC

0.01F

44

AVCC

P3-10

P3-11

AVEE

45

AVEE

P3-12

0.01F

OUTPUT 00

OUTPUT 01

OUTPUT 02

OUTPUT 03

OUTPUT 04

OUTPUT 05

OUTPUT 06

OUTPUT 07

D3

NC

P3-13

P3-14

AD8108/AD8109

AGND

42

AGND

AVEE

AGND

AVCC

AGND

AVEE

AGND

AVCC

AGND

AVEE

AGND

AVCC

AGND

AVEE

AGND

AVCC

AVCC

AVEE

75

41

40

39

75

38

37

36

75

35

34

33

75

32

31

30

75

29

28

27

75

26

25

24

75

23

22

21

75

20

19

18

17

R25

20k

SERIAL MODE
JUMP

0.01F

0.01F

0.01F

0.01F

0.01F

0.01F

0.01F

0.01F

0.01F

0.01F

DVCC

AVEE

AVCC

AVEE

AVCC

AVEE

AVCC

AVEE

AVCC

AVCC

AVEE

Figure 10. Evaluation Board Schematic

–21–REV. A

AD8108/AD8109

Figure 11. Component Side Silkscreen

Figure 12. Board Layout (Component Side)

–22–

REV. A

AD8108/AD8109

Figure 13. Board Layout (Signal Layer)

Figure 14. Board Layout (Power Plane)

–23–REV. A

AD8108/AD8109

Figure 15. Board Layout (Bottom Layer)

–24–

REV. A

AD8108/AD8109

Optimized for video applications, all signal inputs and outputs
are terminated with 75 Ω resistors. Stripline techniques are used
to achieve a characteristic impedance on the signal input and
output lines also of 75 Ω. Figure 16 shows a cross-section of one
of the input or output tracks along with the arrangement of the
PCB layers. It should be noted that unused regions of the four
layers are filled up with ground planes. As a result, the input
and output traces, in addition to having controlled impedances,
are well shielded.

w = 0.008"

(0.2mm)

b = 0.024"

(0.6mm)

a = 0.008"

(0.2mm)

t = 0.00135" (0.0343mm)

h = 0.011325"

(0.288mm)

TOP LAYER

SIGNAL LAYER

POWER LAYER

BOTTOM LAYER

Figure 16. Cross-Section of Input and Output Traces

The board has 16 BNC type connectors: eight inputs and eight
outputs. The connectors are arranged in two crescents around
the device. As can be seen from Figure 11, this results in all
eight input signal traces and all eight signal output traces having
the same length. This is useful in tests such as All-Hostile
Crosstalk where the phase relationship and delay between sig­nals needs to be maintained from input to output.

The three power supply pins AVCC, DVCC, and AVEE should
be connected to good quality, low noise, ±5 V supplies. Where
the same ±5 V power supplies are used for analog and digital,
separate cables should be run for the power supply to the evalu­ation board’s analog and digital power supply pins.

As a general rule, each power supply pin (or group of adjacent
power supply pins) should be locally decoupled with a 0.01 µF
capacitor. If there is a space constraint, it is more important to
decouple analog power supply pins before digital power supply
pins. A 0.1 µF capacitor, located reasonably close to the pins,
can be used to decouple a number of power supply pins. Finally
a 10 µF capacitor should be used to decouple power supplies as
they come on to the board.

Controlling the Evaluation Board from a PC

The evaluation board include Windows-based control software
and a custom cable that connects the board’s digital interface to
the printer port of the PC. The wiring of this cable is shown in
Figure 17. The software requires Windows 3.1 or later to oper­ate. To install the software, insert the disk labeled “Disk #1 of
2” in the PC and run the file called SETUP.EXE. Additional
installation instructions will be given on-screen. Before begin­ning installation, it is important to terminate any other Windows
applications that are running.

MOLEX 0.100" CENTER

CRIMP TERMINAL HOUSING

RESET

CLK

CE

UPDATE

DATA IN

DGND

D-SUB-25

2
3
4
5

6
25

1

6

MOLEX

TERMINAL HOUSING

3
1
4
5

2
6

SIGNAL

CE
RESET
UPDATE

DATA IN

CLK

DGND

D-SUB 25 PIN (MALE)

1

14

25

13

EVALUATION BOARD PC

Figure 17. Evaluation Board-PC Connection Cable

–25–REV. A

AD8108/AD8109

When you launch the crosspoint control software, you will be
asked to select the printer port you are using. Most modern PCs
have only one printer port, usually called LPT1. However, some
laptop computers use the PRN port.

Figure 18 shows the main screen of the control software in its
initial reset state (all outputs off). Using the mouse, any input
can be connected with one or more outputs by simply clicking
on the appropriate radio buttons in the 8 × 8 on-screen array.
Each time a button is clicked on, the software automatically
sends and latches the required 32-bit data stream to the evaluation
board. An output can be turned off by clicking the appropriate
button in the Off column. To turn off all outputs, click on RESET.

The software offers volatile and nonvolatile storage of configura­tions. For volatile storage, up to two configurations can be

stored and recalled using the Memory 1 and Memory 2 Buffers.
These function in an identical fashion to the memory on a
pocket calculator. For nonvolatile storage of a configuration, the
Save Setup and Load Setup functions can be used. This stores
the configuration as a data file on disk.

Overshoot on PC Printer Ports’ Data Lines

The data lines on some printer ports have excessive overshoot.
Overshoot on the pin that is used as the serial clock (Pin 6 on
the D-Sub-25 connector) can cause communication problems.
This overshoot can be eliminated by connecting a capacitor
from the CLK line on the evaluation board to ground. A pad
has been provided on the solder-side of the evaluation board to
allow this capacitor to be soldered into place. Depending upon
the overshoot from the printer port, this capacitor may need to
be as large as 0.01 µF.

Figure 18. Evaluation Board Control Panel

–26–

REV. A

0.030 (0.75)

0.020 (0.50)

SEATING

PLANE

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

80-Lead Plastic LQFP

(ST-80A)

0.559 (14.20)

0.063 (1.60)
MAX

1

0.543 (13.80)

0.476 (12.10)

0.469 (11.90)

TOP VIEW

(PINS DOWN)

6180

60

0.476 (12.10)

0.469 (11.90)

0.559 (14.20)

0.543 (13.80)

AD8108/AD8109

0.003 (0.08)
MAX

0.006 (0.15)

0.002 (0.05)

0.057 (1.45)

0.053 (1.35)

CONTROLLING DIMENSIONS ARE IN MILLIMETERS (mm)

ENGLISH DIMENSIONS (INCHES) ARE APPROXIMATE CONVERSIONS

20

21

0.020 (0.50)
BSC

0.011 (0.27)

0.007 (0.17)

41

40

–27–REV. A

AD8108/AD8109

Revision History

Location Page

Data Sheet changed from REV. 0 to REV. A.

Universal change in nomenclature from MQFP to LQFP

Comment added to OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

C01068–0–1/02(A)

–28–

PRINTED IN U.S.A.

REV. A