Philips oq2538hp DATASHEETS

INTEGRATED CIRCUITS

DATA SH EET

OQ2538HP; OQ2538U

SDH/SONET STM16/OC48 main
amplifiers

Product specification
Supersedes data of 1998 Oct 14
File under Integrated Circuits, IC19

2000 Sep 29

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

FEATURES

• Differential 100 Ω outputs for direct connection to
Current-Mode Logic (CML) inputs

• Wide bandwidth (3 GHz)

• 48.5 dB limiting gain

• Noise figure typically 11 dB

• Automatic offset compensation

• Input level-detection circuitsfor Automatic Gain Control

(AGC) and Loss Of Signal (LOS) detection

• Low power dissipation (typically 270 mW)

• Single −4.5 V supply voltage

• Low cost LQFP48 plastic package.

APPLICATIONS

• Main amplifier in Synchronous Digital Hierarchy (SDH)
andSynchronousOpticalNetwork(SONET)systems for
short, medium and long haul optical transmission

• Level detector for laser diode control loops

• Wideband RF gain block with internal level detectors.

GENERAL DESCRIPTION

The OQ2538HP is a limiting amplifier IC intended for use
as the main amplifier in 2.5 Gbits/s Non-Return to Zero
(NRZ) transmission systems (SDH/SONET).

Comprised of four amplifier stages with a total gain of

48.5 dB, it provides for a wide input signal dynamic range
at a constant CML-compatible output level.

Two level-detection circuits are provided for monitoring
AGC and LOS input signal levels. An internal automatic
offset compensation circuit eliminates offset in the
amplifier chain.

ORDERING INFORMATION

TYPE

NUMBER

OQ2538HP LQFP48 plastic low profile quad flat package; 48 leads; body 7 × 7 × 1.4 mm SOT313-2
OQ2538U − bare die; dimensions 2070 × 2070 × 380 µm −

NAME DESCRIPTION VERSION

PACKAGE

2000 Sep 29 2

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

BLOCK DIAGRAM

handbook, full pagewidth

INQ

V

EE

3

AGC

43

A

B

8

IN

6

AMP A AMP B AMP C AMP D

reference

BAND GAP

REF

CAPA

voltage
for all cells

22 44

4521

COFF COFFQ GND

OQ2538HP

MGE745

AGCDC

19

LOS

18

LOSDC

32

OUT

30

OUTQ

Fig.1 Block diagram.

2000 Sep 29 3

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

PINNING

SYMBOL

V

EE

PIN

(OQ2538HP)

1, 12, 13, 24, 25,

36, 37, 48

n.c. 2,11, 14, 15, 23,

PAD

(OQ2538U)

2, 3, 11, 12, 28,

(2)

29

(3)

20, 22

(1)

TYPE

S negative power supply

− not connected

DESCRIPTION

26, 27, 35, 38,

40, 46, 47
AGC 3 30 O rectifier A output
GND 4, 5, 7, 9, 10, 16,

17, 20, 28, 29,
31, 33, 34, 39,

41, 42

1, 4, 5, 8, 13, 14,

16, 18, 19, 21,
23, 24, 31, 32,

(2)

34, 36

S ground

INQ 6 33 I main amplifier inverting input
IN 8 35 I main amplifier input
LOSDC 18 6 O rectifier B reference output
LOS 19 7 O rectifier B output
REF 21 9 O band gap reference
CAPA 22 10 A pin for connecting band gap reference decoupling

capacitor
OUTQ 30 15 O main amplifier inverted output
OUT 32 17 O main amplifier output
AGCDC 43 25 O rectifier A reference output
COFFQ 44 26 A pin for connecting automatic offset control capacitor

(return)
COFF 45 27 A pin for connecting automatic offset control capacitor

Notes

1. Pin type abbreviations: O = Output, I = Input, S = power Supply and A = Analog function.

2. All GND and V

pads must be bonded; do not leave one single GND or VEE pad unconnected!

EE

3. Pads denoted ‘n.c.’ should not be connected. Connections to these pads degrade device performance.

2000 Sep 29 4

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

handbook, full pagewidth

EE

n.c.

n.c.
47

COFF

COFFQ

46

45

44

V
48

GND

AGCDC
43

42

GND
41

n.c.
40

GND
39

n.c.
38

EE

V

V

EE

n.c.
AGC
GND
GND

INQ

GND

GND
GND

n.c.

V

EE

V

1
2
3
4
5
6
7

IN

8

9
10
11
12

13

14

EE

n.c.

V

15

n.c.

OQ2538HP

16

17

GND

GND

18

19

LOS

LOSDC

20

GND

21

REF

22

CAPA

23

n.c.

36

EE

35

n.c.

34

GND

33

GND

32

OUT

31

GND
OUTQ

30
29

GND
GND

28
27

n.c.

26

n.c.
V

25

EE

24 37

MGE744

EE

V

Fig.2 Pin configuration.

2000 Sep 29 5

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

FUNCTIONAL DESCRIPTION

TheOQ2538HPis comprised of four DC-coupled amplifier
stages along with additional circuitry for offset
compensation and level detection.

The first amplifier stage contains a modified
Cherry/Hooper amplifying cell with high gain
(approximately 20 dB) and a wide bandwidth. Special
attention is paid to minimizing the equivalent input noise at
this stage, thus reducing the overall noise level. Additional
feedback is applied at the second and third stages,
improving isolation and reducing the gain to 14 dB per
stage. The last stage is an output buffer, a unity gain
amplifier, with an output impedance of 100 Ω.

The total gain of the OQ2538HP amounts to 48.5 dB, thus
providing a constant CML-compatible output signal over a
wide input signal range.

Two rectifier circuits are used to measure the input signal
level. Two separate RF preamplifiers are used to generate
thevoltagegainneededtoobtainasuitablerectifieroutput
voltage. For rectifier A the gain is approximately 18 dB, for
rectifier B it is about 14 dB. The output of rectifier A can be
used for AGC at the preamplifier stage in front of the
OQ2538HP. The output of rectifier B can be used for LOS
detection. There is a linear relationship between the
rectifier output voltage and the input signal level provided
the amplifiers are not saturated.

REF and CAPA band gap output and decoupling capacitance

To reduce band gap noise levels, a 1 nF decoupling
capacitor on CAPA is recommended. Since the band gap
isreferenced to the negative power supply, the decoupling
capacitor should be connected between CAPA and V

EE

The band gap voltage is present on pin REF for test
purposes only. It is not intended to serve as an external
reference.

RF input and output connections

Striplines, or microstrips, with an odd mode characteristic
impedance of Z

=50Ω must be used for the

o(odd)

differential RF connections on the PCB. This applies to
both the input signal pair IN and INQ and to the output
signal pair OUT and OUTQ. The two lines in each pair
should have the same length.

RF input matching circuit

The input circuit for pins IN and INQ contains internal
100 Ω resistors decoupled to ground via an internal
common mode 6 pF capacitor. The topology is depicted in
Fig.3.

.

Because the four gain stages are DC-coupled and provide
a high overall gain, the effect of the input offset can be
considerable. The OQ2538HP features an internal offset
compensation circuit for eliminating the input offset.
The bandwidth of the offset control loop is determined by
an external capacitor.

COFF and COFFQ offset compensation

Automatic offset compensation eliminates the input offset
of the OQ2538HP. This offset cancellation influences the
low frequency gain of the amplifier stages. With a
capacitance of 100 nF between COFF and COFFQ the
loop bandwidth will be less than 1.5 kHz, small enough to
haveno influence on amplifier gain over the frequencies of
interest. If the capacitor was omitted, the loop bandwidth
would be greater than 30 MHz, which would influence the
input signal gain. The loop bandwidth can be calculated
from the following formula:

1

×

ext

(1)

where C

f

=

------------------------------------------------

loop

2π 1250Ω× C

is the capacitance connected between COFF

ext

and COFFQ.

handbook, halfpage

6 pF

100 Ω

IN INQ

GND

100 Ω

MGM114

Fig.3 RF input topology.

2000 Sep 29 6

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

An external 200 Ω resistor between IN and INQ is
recommended in order to match the inputs to a differential
transmission line, coupled microstrip or stripline with an
odd mode impedance Z

handbook, halfpage

differential line

Z

= 50 Ω

o(odd)

=50Ω, as shown in Fig.4.

o(odd)

22 nF

200 Ω

22 nF

IN

INQ

MGM115

Fig.4 Differential input matching.

For single-ended excitation, separate matching networks
on IN and INQ, as depicted in Fig.5, achieve optimum
matching.Careshouldbetaken to avoid DC loading, since
the OQ2538HP controls its own DC input voltage.
The resistors on the unused input INQ may be combined
for convenience.

RF output matching circuit

Matchingofthemainamplifieroutputs,OUTandOUTQ,is
not mandatory. In most applications, the receiving end of
thetransmissionlinewillbe properly matched, so very little
reflection will occur. Matching the transmitting end to
absorb these reflections is only recommended for very
sensitive applications. In such cases, 100 Ω pull-up
resistors should be connected from OUT and OUTQ to
ground, as close as possible to the IC pins. These
matching resistors will not be needed in most applications,
however. The output circuit of the OQ2538HP is depicted
in Fig.6. For more information see

AN96051”

handbook, halfpage

describing the OM5801 STM16 demo board.

100 Ω 100 Ω

OUT

“Application Note

GND

OUTQ

MGM117

Fig.6 RF output topology.

handbook, halfpage

22 nF

100 Ω

transmission line

Zo = 50 Ω

50 Ω

22 nF

IN

INQ

22 nF

100 Ω

22 nF

MGM116

Fig.5 Single-ended input matching.

In both cases, the essence of good matching is the equity
of the circuitry on both input pins. The impedance seen on
pins IN and INQ should be as equal as possible. For more
information see

“Application Note AN96051

” describing

the OM5801 STM16 demo board.

2000 Sep 29 7

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

RF gain and group delay measurements

The measurement set-up shown in Fig.7 was used to
measurethe single-ended small signal gain as specified in
Chapter “Characteristics”. Since the network analyzer can
only perform single-ended measurements, the
single-ended matching scheme described above is used
to match the inputs of the OQ2538HP to 50 Ω. For greater
accuracy, the outputs are also matched. The gain
measured with this set-up is denoted by S21. Graphs of
typical S21 and group delay characteristics are shown in
Figs 8 and 9. The OQ2538HP test PCB used for these
measurements can be supplied on request.

Although the differential voltage gain of the OQ2538HP
cannotbemeasureddirectly,it can be calculated from S21.
The differential voltage gain is 6 dB greater than the
measured S21 value, typically 46 dB (40 + 6 dB). If the
100 Ω matching resistors on the output are omitted, the
differential voltage gain is increased by a further 2.4 dB,
typically to 48.4 dB. This is due to the fact that the output
load is increased from 25 to 33 Ω, so the output voltage is
increased by a factor of 1.32 (2.4 dB).

When performing S21measurements make sure the input
power level is around −50 dBm, as indicated in Fig.7
(port 1 of the network analyzer). For correct measurement
results the OQ2538 should not be limiting the input signal,
but operate in its linear region. This can be achieved by
using a very small input signal level of −50 dBm.

handbook, full pagewidth

50 Ω SMA

termination

Zo = 50 Ω

50 Ω semi rigid

50 Ω semi rigid

6 GHz NETWORK ANALYZER

S-PARAMETER TEST SET

P = 50 dBm

PORT 1 PORT 2

OQ2538HP

IN

INQ

V

test PCB

= −4.5 V

EE

OUT

OUTQ

100 Ω 100 Ω

100 Ω

100 pF

100 pF

100 Ω

Fig.7 S21 and group delay measurement set-up.

Zo = 50 Ω

50 Ω semi rigid

50 Ω semi rigid

50 Ω SMA
termination

MGM111

2000 Sep 29 8

Philips Semiconductors Product specification

SDH/SONET STM16/OC48 main amplifiers OQ2538HP; OQ2538U

S21 log MAG

handbook, full pagewidth

40 dB

MGM160

(1)

(3)

(4)

(2)

start: 30 kHz

Vertical scale 6 dB/division.
Linear frequency sweep; start: 30 kHz; stop: 6 GHz.
(1) 41.603 dB; 1 GHz.
(2) 38.633 dB; 3.45 GHz.
(3) 41.291 dB; 2 GHz.
(4) 41.386 dB; 2.5 GHz.

Fig.8 S21 characteristic, measured on the OQ2538HP test PCB.

2000 Sep 29 9

stop: 6 GHz