ADA4625-2ARDZ-R7 datasheet(27/35 Pages) AD

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Data Sheet

ADA4625-1/ADA4625-2

Rev. A | Page 27 of 35

PLLs in which the loop gain passes through 0 dB above the R2C2

zero and below the R2C3 pole and R3C4 pole are stable. At low

charge pump currents, the loop gain passes through zero above

R2C2 zero. At high charge pump currents, the loop gain passes

through zero below the R2C3 pole and R3C4 pole (see Figure 97).

LOG FREQUENCY

0dB FULL LOOP GAIN

LOW CURRENT

0dB FULL LOOP GAIN

HIGH CURRENT

LOOP FILTER GAIN

LOOP WITHOUT FILTER, HIGH CURRENT

LOOP WITHOUT FILTER, LOW CURRENT

LOOP FILTER

FULL LOOP, HIGH CURRENT

FULL LOOP, LOW CURRENT

Figure 97. Gain vs. Frequency of PLL and Loop Filter

Figure 98 shows the measured phase noise vs. frequency offset

from 12 GHz carrier for different charge pump currents (ICP).

Generally, most operations have a charge pump current of

2.5 mA and below. Refer to the UG-383 User Guide for details

on running these tests and setting up the software required.

–180

–160

–140

–120

–100

–80

–60

10k

100k

10M

100M

FREQUENCY OFFSET FROM 12GHz CARRIER (Hz)

ICP = 4.7mA

ICP = 2.5mA

ICP = 0.31mA

Figure 98. Phase Noise vs. Frequency Offset from 12 GHz Carrier for Different

Charge Pump Currents (ICP)

The Analog Devices simulation tool, ADIsimPLL, allows the

design and simulation of PLL loop filter topologies and has a

library of Analog Devices op amps built in. The simulation tool

accurately predicts PLL closed-loop phase noise and is able to

model the effect of op amp noise along with the noise of the

other PLL loop components. For more information about the

ADIsimPLL design tools, refer to www.analog.com/ADIsimPLL.

TRANSIMPEDANCE AMPLIFIER

The ADA4625-1 is an excellent choice for low noise transimpe-

dance amplifier (TIA) applications. While its low voltage and

current noise maximize signal-to-noise ratio (SNR), its low

voltage offset and input bias current minimize the dc error at

the amplifier output. Having a true ground sense capability, the

ADA4625-1 is ideal for single-supply operation. In addition, its

rail-to-rail output swing allows the detection and amplification

of a wide range of input current signals. Figure 99 shows the

ADA4625-1 as a current to voltage (I-V) converter with an

electrical model of a photodiode.

–

VOUT

RSH = 1011Ω

ADA4625-1

Figure 99. Equivalent TIA Circuit

Photodiodes can operate in either photovoltaic mode (zero

bias) or photoconductive mode (with an applied reverse-bias

across the diode). Mode selection depends on the speed and

dark current requirements of the application and the choice of

photodiode. In photovoltaic mode, the dark current is at a

minimum and is preferred for low frequency and/or low light

level applications (that is, PN photodiodes). Photoconductive

mode is better for applications that required faster and linear

responses (that is, PIN photodiodes); however, the tradeoffs

include increases in dark and noise currents.

The following transfer function describes the transimpedance

gain of Figure 99:

OUT

(1)

where:

VOUT is the desired output dc voltage of the op amp.

ID is the output current of the photodiode.

RF and CF are the feedback resistor and capacitor. The parallel

combination of RF and CF sets the signal bandwidth.

s is the s plane.

Set RF such that the maximum attainable output voltage

corresponds to the maximum diode output current. Because

signal levels increase directly with RF, while the noise due to RF

increases with the square root of the resistor value, employing

the full output swing maximizes the SNR.

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