ATA12001

AGC Transimpedance Amplifier SONET OC-24 FEATURES · · · · · Single +5 Volt Supply Automatic Gain Control -31 dBm Sensitivity 0 dBm Optical Overload 1100 MHz Bandwidth 925 um VDD1 ATA12001 PRELIMINARY DATA SHEET-Rev 4 VDD2 GND GND 1992 4E IIN APPLICATIONS · · · · SONET OC-24 (1 Gb/s) Receiver Low Noise RF Amplifier GND GND GND GND CBY CBY GND CAGC VOUT GND BISDN HIPPI 1250 um D1C PRODUCT DESCRIPTION The ANADIGICS ATA12001 is a 5V low noise transimpedance amplifier with AGC designed to be used in 1Gb/s fiber optic links. The device is used in conjunction with a photodetector (PIN diode or avalanche photodiode) to convert an optical signal into an output voltage. The ATA12001 offers a bandwidth of 1100MHz and a dynamic range of 31dB. It is manufactured in a GaAs MESFET process and is available in bare die form. VDD1 AGC 4K 70K VDD2 CAGC GND or neg. Supply IIN - 35 VGA 4pF + 0.8 VOUT GND PATENT PENDING BY Photodetector Cathode must be connected to IIN for proper AGC operation C Figure 1: Equivalent Circuit 08/2001 ATA12001 VDD2 VDD1 GND GND 1992 925 um 4E IIN GND GND GND GND CBY CBY VOUT GND CAGC GND 1250 um Figure 2: Bonding Pad Layout Table 1: ATA12001 Pad Description PAD V DD1 V DD2 IIN VOUT C AGC C BY D ESC R IPTION V DD1 V DD2 TIA Input C urrent TIA Output Voltage External AGC C apaci tor Input Gai n Stage Bypass C apaci tor C OMMEN T Posi ti ve supply for i nput gai n stage Posi ti ve supply for second gai n stage C onnect detector cathode for proper operati on Requi res external D C block 70K * C AGC = AGC RC Ti me C onstant >56 pF ELECTRICAL CHARACTERISTICS Table 2: Absolute Maximum Ratings V DD1 V DD2 IIN TA TS 7.0 V 7.0 V 5 mA Operati ng Temp. - 40 °C to 125 °C Storage Temp. - 65 °C to 150 °C Stresses in excess of the absolute ratings may cause permanent damage. Functional operation is not implied under these conditions. Exposure to absolute ratings for extended periods of time may adversely affect reliability. 2 PRELIMINARY DATA SHEET - Rev 4 08/2001 ATA12001 Table 3: Electrical Specifications (1) (TA = 25°C, VDD =+5.0V + 10%, CDIODE + CSTRAY = 0.5 pF, Det. cathode to IIN) PAR AMETER Transresi stance (RL= ¥ ,IDC=56pF) are essential for good high frequency and low noise performance. The power supply bypass capacitors should be mounted on or connected to a good low inductance ground plane. General Layout Considerations Since the gain stages of the transimpedance amplifier have an open loop bandwidth in excess of 1.5 GHz, it is essential to maintain good high frequency layout practices. To prevent oscillations, a low inductance RF ground plane should be made available for power supply bypassing. Traces that can be made short should be made short, and the utmost care should be taken to maintain very low capacitance at the photodiode-TIA interface (IIN), as excess capacitance at this node will cause a degradation in bandwidth and sensitivity (see Bandwidth vs. CT curves). CT = 0.5 pF 1.3 Bandwidth (GHz) 1.2 1.1 1.0 0.9 VDD = 5.5 V VDD = 5.0 V VDD=4.5 V 10 60 85 -40 Temperature (OC) Figure 4: Bandwidth vs. Temperature 4 PRELIMINARY DATA SHEET - Rev 4 08/2001 ATA12001 BANDWIDTH vs. CT 1800 1.7 Bandwith (MHz) 1600 1400 1200 1000 800 600 VDD=5.5 V 1.5 1.4 VDD = 5.5 V VDD = 5.0 V VDD=4.5 V RF I 1.3 1.2 VDD = 4.5 V 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Figure 5: Bandwidth vs. CT - 2.2 - 1.7 IIN 5 1.1 1.0 - 1.2 - 0.7 - 0.2 CT(pF) IIN (mA DC) Figure 7: Bandwidth vs. IIN Note: All performance curves are typical @ TA =25°C unless otherwise noted. IIN Connection (refer to the equivalent circuit diagram) Bonding the detector cathode to IIN (and thus drawing current from the ATA12001) improves the dynamic range. Although the detector may be used in the reverse direction for input currents not exceeding 25 mA, the specifications for optical overload will not be met. 2.0 1.8 1.6 1.4 I IIN 12001 50 1.2 1.0 0.8 0.6 0.4 Transimpedance (K Ohm) VOUT Connection The output pad should be connected via a coupling capacitor to the next stage of the receiver channel (filter or decision circuits), as the output buffers are not designed to drive a DC coupled 50 ohm load (this would require an output bias current of approximately 36 mA to maintain a quiescent 1.8 Volts across the output load). If VOUT is connected to a high input impedance decision circuit (>500 ohms), then a coupling capacitor may not be required, although caution should be exercised since DC offsets of the photo detector/TIA combination may cause clipping of subsequent gain or decision circuits. heavy AGC Output Collapse 3.4 3.2 3.0 2.9 2.7 2.5 2.4 2.2 2.0 1.9 1.7 1.5 1.4 1.2 1.0 0.8 0.7 0.5 0.3 0.2 0.0 VDD = 5.5 V VDD = 4.5 V VDD =5.5 V Linear Region Rf IIN 12001 -2.2 -1.7 -1.2 IIN (mA DC) -0.7 -0.2 VOUT Figure 6: Transimpedance vs. IIN -4 VDD=4.5 V -3 -2 -1 IIN(mA DC) Figure 8: VOUT vs. IIN PRELIMINARY DATA SHEET - Rev 4 08/2001 VOUT(Volts) BANDWIDTH (GHz) 5 B(3dB) ≈ A / 2 π RF (CN + CT) 1.6 ATA12001 Input Offset Voltage DC 1.9 1.85 1.8 1.75 1.7 1.65 1.6 1.55 1.5 - 40 10 60 85 VDD = 5.5 V VDD = 5.0V (see VOUT vs IIN figure) from the TIA and determining the point of output voltage collapse. Also the input node virtual ground during “heavy AGC” is checked to verify that the linearity (i.e. pulse width distortion) of the amplifier has not been compromised. Measurement of Input Referred Noise Current The “Input Noise Current” is directly related to sensitivity . It can be defined as the output noise voltage (Vout), with no input signal, (including a 1 GHz lowpass filter at the output of the TIA) divided by the AC transresistance. 10 VDD = 4.5V Temperature (C) Figure 9: Input Offset Voltage vs. Temperature CBY Connection The CBY pad must be connected via a low inductance path to a surface mount capacitor of at least 56 pF (additional capacitance can be added in parallel with the 56 pF or 220 pF capacitors to improve low frequency response and noise performance). Referring to the equivalent circuit diagram and the typical bonding diagram, it is critical that the connection from CBY to the bypass capacitor use two bond wires for low inductance, since any high frequency impedance at this node will be fed back to the open loop amplifier with a resulting loss of transimpedance bandwidth. Two pads are provided for this purpose. Sensitivity and Bandwidth In order to guarantee sensitivity and bandwidth performance, the TIA is subjected to a comprehensive series of tests at the die sort level (100% testing at 25 oC) to verify the DC parametric performance and the high frequency performance (i.e. adequate |S21|) of the amplifier. Acceptably high |S21| of the internal gain stages will ensure low amplifier input capacitance and hence low input referred noise current. Transimpedance sensitivity and bandwidth are then guaranteed by design and correlation with RF and DC die sort test results. ndirect Measurement of Optical Overload Optical overload can be defined as the maximum optical power above which the BER (bit error rate) increases beyond 1 error in 10 10 b its. The ATA12001D1C is 100% tested at die sort by a DC measurement which has excellent correlation with an PRBS optical overload measurement. The measurement consists of sinking a negative current 6 9 8 Hz 7 6 5 4 3 -0.1 1 10 CT RF 50Ω pA/ CT = 1.0pF CT = 0.5pF 100 1000 Figure 10: Input Referred Noise Spectral Density Input Referred Noise in (nA RMS) Input referred noise test circuit 16 25dB VDD = 4.5 V 100 MHz LPF 15 14 13 12 11 10 -40 0 0.5pF TIA VDD = 5.5V η(dBm) = 10 LOG 6500in R 40 80 Temperature (OC) FIgure 11: Input Referred Noise vs Temperature PRELIMINARY DATA SHEET - Rev 4 08/2001 ATA12001 AGC Capacitor It is important to select an external AGC capacitor of high quality and appropriate size. The ATA12001D1C has an on-chip 70 KW resistor with a shunt 4.5 pF capacitor to ground. Without external capacitance the chip will provide an AGC time constant of 315 nS. For the best performance in a typical OC-24 SONET receiver, a minimum AGC capacitor of 56pF is recommended. This will provide the minimum amount of protection against pattern sensitivity and pulse width distortion on repetitive data sequences during high average optical power conditions. Conservative design practices should be followed when selecting an AGC capacitor, since unit to unit variability of the internal time constant and various data conditions can lead to data errors if the chosen value is too small. Phase Response At frequencies below the 3dB bandwidth of the device, the transimpedance phase response is characteristic of a single pole transfer function (as shown in the Phase vs Frequency curve). The output impedance is essentially resistive up to 1200 MHz. Phase (IIN to VOUT) 180 Degrees 200 RF 220 240 IIN 0.5pF VOUT 50 Ω 50 100 150 Frequency (MHz) Figure 12: Phase (IIN to VOUT) PRELIMINARY DATA SHEET - Rev 4 08/2001 7 ATA12001 ORDERING INFORMATION PAR T N U MB ER ATA12001D 1C PAC K AGE OPTION D 1C PAC K AGE D ESC R IPTION Die ANADIGICS, Inc. 141 Mount Bethel Road Warren, New Jersey 07059, U.S.A Tel: +1 (908) 668-5000 Fax: +1 (908) 668-5132 http://www.anadigics.com Mktg@anadigics.com IMPORTANT NOTICE ANADIGICS, Inc. reserves the right to make changes to its products or to discontinue any product at any time without notice. The product specifications contained in Advanced Product Information sheets and Preliminary Data Sheets are subject to change prior to a product’s formal introduction. Information in Data Sheets have been carefully checked and are assumed to be reliable; however, ANADIGICS assumes no responsibilities for inaccuracies. ANADIGICS strongly urges customers to verify that the information they are using is current before placing orders. WARNING ANADIGICS products are not intended for use in life support appliances, devices, or systems. Use of an ANADIGICS product in any such application without written consent is prohibited. PRELIMINARY DATA SHEET - Rev 4 08/2001 8