ADL5310 dual logarithmic converter with 120dB bandwidth

1 Main Features of ADL5310

ADL5310 is a dual logarithmic converter with two independent channels produced by ADI Corporation of the United States. It has an optimized interface for photoelectric conversion and a logarithmic output with stable temperature characteristics. It also has an output buffer amplifier that can be configured by the user. The slope and intercept of its logarithmic conversion transfer function can be adjusted by the user through external resistors. It can be widely used in gain and absorbance measurement, multi-channel power supply monitoring, universal baseband logarithmic compression, etc.

The ADL5310 has two independent signal channels that can configure transfer function constants (logarithmic slope and intercept) for each channel.

The internal bias circuit is shared by both channels, which is equivalent to two AD8305s with an extended dynamic range (120dB).

ADL5310 uses the precise logarithmic relationship between the diode base-emitter voltage and the collector current to provide an ideal linear transmission structure for photoelectric conversion. The slope of its logarithmic transfer function is initially set to 10mV/dB (200mV/dec), but it can also be adjusted by external resistors and independent buffer amplifiers. The intercept of the transfer function of each channel can be defined by its own reference current. The reference current is generally set to 3μA by connecting the resistor between the VREF (2.5V) pin and the IRF1 (IRF2) pin at the beginning, and the VRDZ pin is connected to the VREF pin to effectively set the X-axis intercept to 1/10000 of the reference current. Generally, when the reference current is 3μA, the intercept is 300pA.

When measuring gain or absorption coefficient, if the input current range of the two channels is different, the ADL5310 can use two independent reference currents. The reference current of the ADL5310 allows the input amplitude to be the entire dynamic input range of the circuit, and the reference input current can be used as a denominator in logarithmic operations. The operating temperature range of the ADL5310 is -40℃ to +85℃. It is provided with a suitable bias current by a high-precision current generator to compensate for the inherent temperature effect of the diode.

ADL5310 is widely used in light source detection of optical communication systems, such as laser control circuits, optical converters, attenuators, amplifiers and system monitoring.

2 Pin Functions of ADL5310

The pin arrangement of ADL5310 is shown in Figure 1, and the pin functions are as follows:

VSUM: protection pin, used to protect the input current of INP1 and INP2, and to automatically adjust the voltage of the input main node;

INP1, INP2: molecular input of channels 1 and 2, from which the photodiode current IPD1 and IPD2 flow, usually connected to the positive electrode of the photodiode;

IRF1, IRF2: Denominator input of channels 1 and 2, from which reference currents IRF1 and IRF2 flow;

VREF: 2.5V reference output voltage;

VPOS: positive power supply, VP-VN≤12V;

VNEG: adjustable negative power supply, this pin is often grounded;

OUT1, OUT2: buffer output of channels 1 and 2;

SCL1, SCL2: buffered amplifier conversion input of channels 1 and 2;

BIN1, BIN2: buffered amplified non-converted input of channels 1 and 2;

LOG1, LOG2: Logarithmic preamplifier output of channels 1 and 2;

ＣＯＭＭ: analog ground;

VRDZ: Intercept conversion reference input, usually connected to VREF, can also be grounded when providing unipolar input.

3 Main Structure of ADL5310

ADL5310 can provide a very convenient interface for fiber management systems, and it can also be used in non-fiber systems. Figure 2 shows the main principle structure of an independent online amplifier. In the figure, the diode current IPD flows through INP1 (INP2), and the voltage of this node is approximately equal to the voltage VSUM of the adjacent protection pin, that is, the voltage of the reference input IRF1 (IRF2), which is generated by the tiny offset voltage of the JEF type op amp. Transistor Q1 can convert the current IPD into a corresponding logarithmic current. Generally, in the case of a single power supply, a limited positive voltage VSUM should be connected to bias the collector of Q1. In ADL5310, VSUM is built-in to 0.5V (1/5 of the reference voltage 2.5V of the VREF pin).

4 Application Analysis

4.1 Noise Analysis

When estimating the intercept stability, any temperature variation of IRF1 (IRF2) should be taken into account. Also, if the value of IRF1 (IRF2) is small, the overall noise caused by temperature will increase. In fixed intercept applications (circuit shown in Figure 3), do not use a large reference current to avoid compressing the dynamic cutoff range of small currents under single-supply conditions. However, it is necessary to add a capacitor between VSUM and ground to reduce the noise at this point and reduce the interaction between channels to help provide the overall reference current.

In addition, any input terminal and reference terminal (INP1, INP2, IRF1, IRF2) should have a compensation network composed of resistors and capacitors. The connection capacitance of the light-emitting diode and the network capacitance of the input system will form a large pole in the transfer function that varies with the input current. Synchronizing the RC network at a certain frequency can reduce this pole, and inserting a zero point can also compensate for another inherent pole of the input system to stabilize the entire system. Generally speaking, a 1nF and 1kΩ RC network is suitable for any photodiode interface. However, if a photodiode with a larger dynamic range is used, or when the input path is very long, a larger capacitor needs to be used to ensure system stability. A 4.7nF and 2kΩ filter device needs to be connected between IRF1 (IRF2) and ground. In demanding places (such as compensation networks), temperature-stabilized devices are also used. Y5V chip capacitors should be avoided due to their low temperature stability.

The adjustable capacitor connecting pin LOG1 (LOG2) and ground and the 5kΩ resistor at the pin constitute a single-pole low-pass filter. When IPD is very small, the filter is beneficial to suppress output noise, while the multi-pole filter will be better in suppressing the overall noise.

4.2 Minimum crosstalk

Integrating two high-dynamic-range logarithmic converters on a single IC chip also brings with it the potential problem of isolating the channels from each other. To keep crosstalk within acceptable limits, strict power supply bypassing (also required for overall system stability) and careful layout should be employed.

Although the bias protection circuit can improve the matching between channels and reduce power consumption, it is a crosstalk source that must be reduced. Since VSUM is the voltage reference value shared by the two input systems, a 1nF capacitor must be connected between the pin and the ground to achieve the bypass effect. At low current (less than 30nA), a 20nF capacitor is required. In this way, the interference of the VSUM pin within the input bandwidth will be controlled by the feedback loop without causing disturbances at the output (except for the small fluctuations in the reference current caused by the voltage changes of the IRF1 and IRF2 pins).

At low currents, if there is no bandwidth limitation, the initial current will be lost. Therefore, the pole frequency at VSUUM (with a typical 16kΩ source resistance) must be set lower than the minimum bandwidth of the system at the minimum current input that may be encountered. The low-frequency noise at the VSUUM pin should also be tracked and controlled by the feedback loop within the bandwidth to reduce the output disturbance caused by the thermal noise of the 16kΩ source resistance at VSUUM. The VSUUM pin is composed of a 10nF capacitor (two 20nF capacitors in parallel) and a 16kΩ source resistance to form a 500Hz pole, and its pole frequency is much smaller than the bandwidth at the minimum input voltage of 3nA.

5 Two-point calibration

Each channel of ADL5310 has its own independent slope and intercept, which is 200mV/decade?300pA at LOG1 (LOG2). This value is not fixed at the end, and its slope can change by 7.5% due to temperature. For this reason, it is recommended to use a simple correction to achieve the purpose of improving accuracy. If a pair of AD8305 logarithmic operational amplifiers are randomly selected, then to achieve the required accuracy, each channel must be calibrated separately, while the selection of ADL5310 completely improves the matching of the slope and intercept between the two channels without the need for further calibration.