Philips Semiconductors

Product specification

SA5212A

Transimpedance amplifier (140MHz)

1998 Oct 07

13

maximum mean optical power is:

OUT+

OUT–

IN

VIN

NE5212A

R = 560

a. Non-inverting 20dB Amplifier

OUT–

OUT+

IN

NE5212A

R = 560

VIN

b. Inverting 20dB Amplifier

OUT–

OUT+

IN

NE5212A

R = 560

VIN

c. Differential 20dB Amplifier

SD00344

Figure 13. Variable Gain Circuit

P

hcI

avMAX

lq

+

2.3

1.6

= 172

µW or –7.6dBm

This represents the maximum limit attainable with the SA5212A

operating at 200MHz bandwidth, with a half mark/half space digital

transmission at 820nm wavelength.

APPLICATION INFORMATION

Package parasitics, particularly ground lead inductances and

parasitic capacitances, can significantly degrade the frequency

response. Since the SA5212A has differential outputs which can

feed back signals to the input by parasitic package or board layout

capacitances, both peaking and attenuating type frequency

response shaping is possible. Constructing the board layout so that

Ground 1 and Ground 2 have very low impedance paths has

produced the best results. This was accomplished by adding a

ground-plane stripe underneath the device connecting Ground 1,

Pins 8–11, and Ground 2, Pins 1 and 2 on opposite ends of the

SO14 package. This ground-plane stripe also provides isolation

2 and the input photodiode currents to flowing to Ground 1. Without

this ground-plane stripe and with large lead inductances on the

board, the part may be unstable and oscillate near 800MHz. The

easiest way to realize that the part is not functioning normally is to

measure the DC voltages at the outputs. If they are not close to their

quiescent values of 3.3V (for a 5V supply), then the circuit may be

oscillating. Input pin layout necessitates that the photodiode be

physically very close to the input and Ground 1. Connecting Pins 3

and 5 to Ground 1 will tend to shield the input but it will also tend to

increase the capacitance on the input and slightly reduce the

bandwidth.

As with any high-frequency device, some precautions must be

observed in order to enjoy reliable performance. The first of these is

the use of a well-regulated power supply. The supply must be

capable of providing varying amounts of current without significantly

changing the voltage level. Proper supply bypassing requires that a

good quality 0.1

µF high-frequency capacitor be inserted between

pins as possible. Also, the parallel combination of 0.1

µF capacitors

with 10

the ground plane should provide adequate decoupling. Some

applications may require an RF choke in series with the power

supply line. Separate analog and digital ground leads must be

maintained and printed circuit board ground plane should be

employed whenever possible.

BASIC CONFIGURATION

A trans resistance amplifier is a current-to-voltage converter. The

forward transfer function then is defined as voltage out divided by

current in, and is stated in ohms. The lower the source resistance,

the higher the gain. The SA5212A has a differential transresistance

of 14k

Ω typically and a single-ended transresistance of 7kΩ

typically. The device has two outputs: inverting and non-inverting.

The output

voltage in the differential output mode is twice that of the output

voltage in the single-ended mode. Although the device can be used

without coupling capacitors, more care is required to avoid upsetting

the internal bias nodes of the device. Figure 13 shows some basic

configurations.

VARIABLE GAIN

Figure 14 shows a variable gain circuit using the SA5212A and the

SA5230 low voltage op amp. This op amp is configured in a

non-inverting gain of five. The output drives the gate of the SD210

DMOS FET. The series resistance of the FET changes with this

output voltage which in turn changes the gain of the SA5212A. This

circuit has a distortion of less than 1% and a 25dB range, from

-42.2dBm to -15.9dBm at 50MHz, and a 45dB range, from -60dBm

SD210

OUT+

OUT–

0–5V

IN

+5V

10k

2.4k

0–1V

51

NE5212A

VCC

RFOUT

RFIN

0.1

µF

SD00345

Figure 14. Variable Gain Circuit