Patent Publication Number: US-2017373651-A1

Title: Trans-impedance amplifier arrangement and control module

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 14/849,764 filed on Sep. 10, 2015 which claims priority to GB 1416320.8 filed on Sep. 16, 2014 which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Some embodiments relate to a trans-impedance amplifier arrangement, a control module and a combination thereof. 
     BACKGROUND OF THE INVENTION 
     A trans-impedance amplifier is an amplifier circuit arranged to convert a current into a voltage. The gain of a trans-impedance amplifier is the ratio of the output voltage to the input current. 
     The magnitude of the gain provided by the trans-impedance amplifier can be controlled by a feedback resistor. In some arrangements the feedback resistor is connected between the inverting input and the output of an inverting amplifier with high input impedance. The input impedance of the inverting amplifier causes the input current to flow through the feedback resistor producing the output voltage. 
     SUMMARY 
     According to an embodiment, there is provided a trans-impedance amplifier arrangement comprising: an input configured to receive an output from a photo-detector; a current monitoring circuit configured in use to provide a current monitor signal dependent on a current through the photo-detector; an output configured to output said current monitor signal to a control module, said output further configured to receive control information from said control module. 
     The current monitoring circuit may comprise a current mirror. 
     The output may be further configured to provide said received control information to a control circuit. 
     The control circuit may be configured to compare at least part of said received control information with a predetermined code. 
     The control circuit may be configured to provide configuration information to one or more configurable modules of the trans-impedance amplifier in dependence on said the result of said comparing. 
     The control circuit may comprise a shift register for receiving said control information. 
     The control information may be transmitted by a modulated signal. 
     The output may be further configured to transmit information relating to at least one of configuration and operation of the trans-impedance amplifier to said control module. 
     The trans-impedance amplifier information may be transmitted by the current monitor signal. 
     The trans-impedance amplifier arrangement comprising a selector configured to modulate said current monitor signal to transmit said trans-impedance amplifier information. 
     One or more discrete current values may be added at different times so as not to alter the average current value of the current monitor signal and to provide said trans-impedance amplifier information. 
     The trans-impedance amplifier arrangement may comprise a first current source; a second current source; and a selector configured to connect the output to the first and second current sources dependent on said trans-impedance amplifier information. 
     According an embodiment, there is provided a control module comprising: an input configured, in use, to receive data from a trans-impedance amplifier circuit; and a control circuit configured, in use, to provide control information to said trans-impedance amplifier circuit via an output, said output further configured to receive a current monitor signal from said trans-impedance amplifier circuit. 
     The control information may comprise a control header. 
     The control header may be useable by the trans-impedance amplifier circuit to determine when control information has been received. 
     The control information may comprise configuration information for one or more configurable modules of the trans-impedance amplifier. 
     The control module may be configured to transmit said control information with a modulated signal. 
     One of a voltage and a current may be modulated to provide said control information. 
     The received current monitor signal may comprise an average current signal and one or more discrete current values corresponding to trans-impedance amplifier information. 
     The control module may comprise a voltage selector configured to modulate a voltage of said output by modulating a reference voltage. 
     The control module may comprise an operational amplifier and wherein in use: an inverting input of said operational amplifier is provided to said output; and a non-inverting input of said operational amplifier is modulated in dependence on said control information. 
     The control module may comprise a feedback resistor between the inverting input of said operational amplifier and the output of said operational amplifier. 
     In some embodiments, a compensation current is provided to the inverting input of the operational amplifier so as to substantially cancel out voltage changes at the output of said operational amplifier caused by modulating said non-inverting input. 
     The control module may comprise a circuit configured to receive a current into the inverting input such that the value of the current is not affected by the modulation of the non-inverting input. 
     The control module may comprise a transistor configured to one of draw current from or supply current to the inverting input of said operational amplifier, said transistor being controlled by an output of said operational amplifier; and a load resistor configured to convert the current drawn or supplied by the transistor to a voltage. 
     The control module may comprise a filter configured to filter a signal received at said output so as to separate said trans-impedance amplifier information from said current monitor signal. 
     The filter may comprise at least one of: a filter configured to extract the current monitor signal; and a filter configured to extract a signal corresponding to trans-impedance amplifier information. 
     According to an embodiment there is provided a trans-impedance amplifier as provided by the first embodiment in combination with a control module as described in the second embodiment. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       For a better understanding of some embodiments, reference will be made by way of example only to the accompanying drawings in which: 
         FIG. 1A  shows a trans-impedance amplifier circuit with a current monitor output of some embodiments; 
         FIG. 1B  shows a trans-impedance amplifier circuit with a current monitor output of other embodiments; 
         FIG. 2  shows a trans-impedance amplifier circuit connected to a controlling module; 
         FIG. 3  shows a data waveform and recovered clock signal according to some embodiments; 
         FIG. 4  shows a word of the communication scheme of some embodiments; 
         FIG. 5  shows a trans-impedance amplifier integrated circuit of some embodiments; 
         FIG. 6  shows decoding apparatus according some embodiments; 
         FIG. 7  shows a control module according to some embodiments; 
         FIG. 8  shows a control module configured to send control data to a trans-impedance amplifier according to some embodiments; 
         FIG. 9  shows a control module and a trans-impedance amplifier module configured to transmit data to each other according to some embodiments; 
         FIG. 10  shows a control module configured to separate current monitor information from control related data according to some embodiments; 
         FIG. 11  shows apparatus for modulating the current monitor voltage without affecting the sensing of the current according to some embodiments; and 
         FIG. 12  shows apparatus for modulating the current monitor voltage without affecting the sensing of the current according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A trans-impedance amplifier may be arranged to receive the output current of a photo-detector such as a photodiode. The photo-detector may be used to receive a signal. For example, in some embodiments the signal may be a pulsed light signal from an optical transmission source over an optical fibre. A current monitor output (IMON) may be provided to provide an average current signal that mirrors the average current through the photo-detector. The current monitor output may be provided by a current mirror arranged to mirror a current between two pins connected to the photo-detector. 
     Some photo-detectors may require a finely tuned external bias voltage and as such have one connection to the trans-impedance amplifier and one connection to the external bias circuitry. Accordingly in some embodiments the current monitor may be produced by mirroring current drawn through amplifier circuitry. 
     The current monitor output may be used to measure optical performance of a fibre optic system. The average current of the current monitor output may be used to provide a representation of the average light intensity received at the photo-detector. Accordingly, the performance of the photo-detector may be measured by the current monitor output. 
     This may be for example used to monitor if the optical source has degraded or to characterize the optical source. 
     The trans-impedance amplifier integrated circuit may comprise a single voltage output. 
     In other embodiments, there may be circuitry arranged to provide a pair of voltage complementary outputs where the signal of the first output is the inverse signal of the second output. The difference between these two voltage outputs may be used to calculate the output signal. The two output signals may be in close proximity. This output method may have the advantage that noise may affect both output signals equally. Accordingly, the difference between the two signals would remain unaffected thus reducing the effect of noise on the output. 
     Reference is now made to  FIG. 1A  which shows a trans-impedance amplifier circuit having a photodiode  101  provided between two pins: PinA  102  and PinK  103 . In the configuration of  FIG. 1A , the anode of the photodiode  101  is coupled to PinA  102  whilst the cathode is coupled to PinK  103 . PinA  102  is an input terminal for receiving current from the photodiode  101 . An amplifier  104  is provided between PinA  102  and an output with a feedback resistor  105  between the input and the output of the amplifier  104 . A DC restore circuit  106  is connected between PinA  102  and the negative power supply VSS  107 . A voltage regulator  108  is provided between the positive supply rail VDD  109  and PinK  103 . The voltage regulator  108  comprises a current mirror  110  which is connected to a current monitor output IMON  111 . The average current through the photodiode  112  is provided by the voltage regulator  108  through PinK  103 . Accordingly the current  113  output by the current mirror  110  to the IMON output  111  is a mirror of the average photodiode current  112 . 
     The current monitor output (IMON) is used to provide an average of the photodiode current. This current can be measured and used to provide an indication of average signal intensity. In some embodiments the IMON output may be monitored to detect performance issues. In one embodiment, a drop in the IMON output current may indicate that the photodiode is faulty. In another embodiment, a drop in the IMON output current could be detected and used to identify degradation in performance of a light source. 
     Reference is now made to  FIG. 1B  which shows a trans-impedance amplifier circuit with a current monitor output for an avalanche photodiode (APD). An APD may require a finely tuned bias voltage which may be applied from an external source. Accordingly an APD may not be suitable for connection between PinA and PinK. In the configuration of  FIG. 1B , the anode of the APD  118  is provided to PinA  102  whilst the cathode is provided to an external bias voltage Vapd  116 . An amplifier  104  is provided between PinA  102  and an output with a feedback resistor  105  between the input and the output of the amplifier  104 . A DC restore circuit  114  is provided between PinA  102  and the negative power supply VSS  107 . The DC restore circuit  114  comprises a current mirror  117  which is output to a current monitor output IMON  111 . The average current through the photodiode  119  flows through the DC Restore circuit. Accordingly the current  115  output by the current mirror to the IMON output  111  is a mirror of the average photodiode current  119 . 
     Reference is made to  FIG. 2  which shows an arrangement of some embodiments. A trans-impedance amplifier package  203  is provided. Trans-impedance amplifier package  203  comprises trans-impedance amplifier circuitry  205  coupled to photo-detector  207 . Photo-detector  207  may be configured to receive optical data. The optical data may be provided over a fibre-optic cable. Trans-impedance amplifier circuitry  205  is configured to provide at an output  209  wherein the output  209  corresponds to the received optical data. The output may comprise a single output pin in some embodiments. In other embodiments, the output may be provided by two or more output pins. The output  209  is provided to a control module  201  which may be in communication with one or more circuits. Control module  201  may be configured to route the data received from the output  209  to a destination where said data is needed. Trans-impedance amplifier package  203  provides current mirror output  211  to control module  201 . Accordingly control module  201  may monitor the average current through photo-detector  207 . This average current may be used to estimate the average incident light received at photo-detector  207 . This can be used for any suitable purpose. For example this may be used to indicate performance degradation of the optical source. Control circuit  213  is provided to receive the current monitor information in the control module. 
     Control  201  module may comprise a microprocessor. In other embodiments the control module  201  may comprise a programmable logic device (PLD) or a field programmable gate array (FPGA). In other embodiments the control module  201  may comprise an application specific integrated circuit (ASIC). 
     The control circuit  213  is configured to communicate with trans-impedance amplifier package  203 . This communication may comprise providing control information to the trans-impedance amplifier package. This may allow one or more problems to be addressed, in some embodiments. 
     The control module  201  and the trans-impedance amplifier package  203  may be manufactured by different suppliers. Accordingly, to ensure inter-operability, it may be necessary to produce control module  201  and trans-impedance amplifier package  203  with large tolerances. In other words, to ensure that the trans-impedance amplifier package of one producer works with the control module of another, both may be over engineered. This may lead to overall inefficiency. For example, the supply voltage or the bandwidth of the trans-impedance amplifier may be higher than would be required to successfully operate. In large banks of photo-detectors, this inefficiency may lead to large amounts of unnecessary heat which may be difficult to remove. 
     In some embodiments, control module  201  may be configured to communicate with trans-impedance amplifier package  203  over current monitor output  211 . This removes the need for additional pins on both the control module and the photo-detector. This may allow the photo-detector to use the same package and allow it to communicate with control modules of other manufacturers. 
     The control module may be configured to dynamically adjust the performance of the trans-impedance amplifier package. For example, in one embodiment the control module may be configured to increase the bandwidth of the trans-impedance amplifier package during high transmission periods to allow more data to be transmitted. In another example, the control module may be configured to reduce the bandwidth during low transmission periods to reduce power consumption and/or thermal output. It should be appreciated that bandwidth has been used by way of example and other variables may be increased or reduced by the control module. 
     The control module may be configured to control the trans-impedance amplifier package to specific characteristics such that a single trans-impedance amplifier can be used in a variety of applications. 
     In some embodiments the control module is configured to communicate with the trans-impedance amplifier. In other embodiments the communication may be from the trans-impedance amplifier to the control module. In other embodiments the communication may be two way. 
     Reference is made to  FIG. 3  which shows a waveform of a communication scheme using the current monitor signal pathway to provide communication according to some embodiments. It should be appreciated that the communication scheme is by way of example and other communications schemes may be employed. Data waveform  301  shows the voltage of the current monitor line  211  over time during a communication from the control module. Clock waveform  303  shows a clock signal calculated from the current monitor line. The data may be communicated over current monitor line  211  by pulse width modulation. It should be appreciated that the use of pulse width modulation is by way of example and others methods of modulation and data encoding may be used. Two pulse lengths are used. In one example, a pulse length of 8 μs encodes logic ‘1’ whilst a pulse of less than 3.8 μs encodes logic ‘0’. It should be appreciated that other pulse lengths may be used. It should also be appreciated that the terms logic ‘0’ and logic ‘1’ may be exchanged. Each pulse is followed by a waiting period dependent on the pulse length to the effect that the starts of the pulses are a fixed time apart. In one embodiment, clock signal  303  is calculated from the rising edges of the pulse on current monitor line  211 . This rising edge is converted into a short pulse  305  and is delayed by a time period that falls between the two pulse lengths. In another embodiment, clock signal  303  is calculated from the falling edges of the pulse current monitor line  211  and is used to generate pulses  305 . Clock signal  303  is used to sample data  301 . The delay of the clock pulse causes the data to be sampled as high during a pulse encoding logic ‘1’ and low during a pulse encoding logic ‘0’. In other embodiments data may be sampled as high if during a pulse encoding logic ‘0’ and low during a pulse encoding logic ‘1’.  FIG. 3  shows an example with the current monitor line with two long pulses  307  encoding logic ‘1’ followed by three short pulses encoding logic ‘0’. 
     Since the current monitor signal is provided by a current source, it may be possible to provide a voltage to the current monitor signal line at the control module without affecting the current monitor current signal. Accordingly in some embodiments, the control module may communicate with the trans-impedance amplifier without interrupting the current monitor output but using the same communication pathway. 
     In other embodiments the current monitor output may be converted into a voltage at the monitor pin of the TIA and the control data may be in the form of a current from the control module. In some embodiments the control module and the trans-impedance amplifier may be configured to not output at the same time and instead use a time division approach. 
     Reference is made to  FIG. 4  which shows a word of an example communication scheme of some embodiments. Word  401  comprises control header  405  and control data  403 . Control header  405  comprises a number of bits which allow the trans-impedance amplifier package to determine if a word is received. The control header may be used to indicate the source of the control module to the trans-impedance amplifier. Alternatively or additionally the control header may be used to verify that the signal received is an intentional signal and not a product of noise. The remainder of word  401  is the control data  403 . Control data  403  may comprise a number of bits configured to provide setting information for one or more settings to the trans-impedance amplifier package. Through these control bits, the control module is able to communicate new setting information to the trans-impedance amplifier package. In the example of  FIG. 4 , the word comprises 62 bits of which 24 bits comprise the control header and the remaining 38 bits comprise the control data. It should be appreciated that other sizes of control header and control data may be used. It should be appreciated that the control header does not need to be at either end of the word. In some embodiments the bits of the control header may be distributed amongst the bits of the control data. In other embodiments the entire word may comprise the functionality of both the control header and the control data. For example, the received word may be compared to a list of valid codes. If the word does not match any of the valid codes then it is ignored. If the word does match a valid code then the configuration of the trans-impedance amplifier may be changed in accordance with a configuration specific to the valid code. 
     Reference is made to  FIG. 5  which shows a trans-impedance amplifier integrated circuit of some embodiments. Integrated circuit  501  comprises power supply pin (VDD)  507 , ground pin (GND)  509 , current monitor pin (MON)  511 , monitor polarity pin (MPOL)  513 , bias voltage pin (PK)  503 , photo-detector input pin (PA)  505  and output pins (OUTP and OUTN)  515  and  517 . Photo-detector bias filter  529  provides a bias voltage to PK  503 . A voltage regulator  508  and band gap circuit  510  are provided. The photo-detector input pin PA  505  provides current input from a photo-detector to trans-impedance amplifier (TIA)  519 . The gain of the TIA  519  is controlled by an automatic gain controller (AGC)  523 . The output of TIA  519  is provided to a single end to differential converter (SE 2 DIFF)  525  which is configured to convert the TIA output into a differential signal path. The differential signal path is provided to a current-mode logic (CML) driver  527  which provides outputs  515  and  517 . The outputs of the convertor SE 2 DIFF  525  are further configured to control the AGC  523  to control the gain of TIA  519 . A DC restore control loop  521  is configured to control the DC voltage across the TIA  519  to 0V or some other convenient reference value. Current monitor circuitry  531  is provided to provide the current monitor output  511 . Control logic  533  is provided to receive a control signal from the MPOL pin which may determine the polarity of the current monitor output. A decoder  601  may be provided to receive and decode the control information from the control module. 
     The decoder may be configured to provide configuration information to a number of parts of the trans-impedance amplifier package. This configuration information may be used to set a number of parameters. 
     For example:
         whether the current monitor output sources or sinks current;   enabling of the DC restore loop;   enabling the automatic gain control;   the bandwidth of the SE 2 DIFF; and   ptat (proportional to absolute temperature) settings of the CML driver.       

     It should be appreciated these parameters are listed by way of example and other parameters of the trans-impedance amplifier may alternative or additionally be configured. 
     Reference is made to  FIG. 6  which shows decoding apparatus according some embodiments. It should be appreciated that description of the decoding apparatus is by way of example and alternative methods of decoding the control information may be used. Decoding circuitry  601  comprises a load shift register  603 . Load shift register  603  is provided with the data input from the current monitor line and the recovered clock signal. The load shift register comprises two regions: a test settings region  605  and a test key region  607 . Load shift register  603  is configured to clock in data from one end. The individual registers of load shift register  603  have outputs. The outputs of the test settings registers are provided to settings register  609 . The outputs of the test key registers are provided to decoder  611 . Decoder  611  is configured to detect if the data stored in the test key region  607  of load shift register  603  matches a predetermined pattern. Upon detecting a predetermined pattern decoder  611  is configured to provide a clock pulse on a clock signal input to settings register  609 . When the clock pulse is received at settings register  609  the data from the test settings region  605  of load shift register  603  are clocked into settings register. Accordingly, decoding circuit  601  determines when a valid word has been received by matching the test key to a predetermined pattern and stores the settings information in the settings register. The settings register may be used to provide the settings information to other areas of the trans-impedance amplifier circuit. 
     Reference is made to  FIG. 7  which shows a control module according to some embodiments. Control module  701  comprises an input  707  for receiving data from the trans-impedance amplifier package and an input  709  for receiving the current monitor signal from the trans-impedance amplifier package. The data received at input  707  are provided to data controller  711  which handles the data from the trans-impedance amplifier. The current monitor information received at input  709  is provided to current monitor controller  705  which handles the current monitor signal. Control circuit  703  provides the control information which is output to the trans-impedance amplifier package via the current monitor input  709 . 
     Reference is made to  FIG. 8  which shows an embodiment showing the TIA module of  FIG. 1A  connected to control module  801 . The TIA module comprises comparator  811  which provides digital data to the control logic  513  sent from the control module  801  in the form of different voltage levels. These data correspond to the control information sent by the control module. Control module  801  comprises an input  803  connected to the IMON output  111  of the TIA module. A resistor  805  is provided between input  803  and ground or any convenient reference voltage to provide a current to voltage conversion. Control module  801  comprises an analogue to digital converter  807  which is configured to convert the voltage across resistor  805  to a digital value. This digital value corresponds to the average current monitor signal provided by the TIA. 
     Voltage selector  809  is provided in the control module to allow modulation of the current monitor line to provide control information to the TIA module. The voltage selector may select between any number of voltages. In one embodiment a first voltage is used to correspond to logic ‘1’ and second voltage is used to correspond to logic ‘0’, The voltage selector may optionally have a not connected state wherein no control information is transmitted. 
     In addition to the control module sending configuration and control data to the TIA module, it may also be desirable for the TIA to send data to the control module in addition to the current monitor signal. These data may contain information confirming the present configuration state of the TIA module after some change has been requested, or any other information which may be used to optimise management of the complete system. 
     Reference is made to  FIG. 9  which shows the embodiment of  FIG. 8  with the addition of example apparatus for allowing bidirectional communication. The TIA comprises selector  901  which allows the IMON output  111  to be connected to either a first current source  903 , a second current source  905  or neither. The first current source  903  is configured to sink current from the IMON output whilst the second current source  905  is configured to source current. The selection of current sources  903  and  905  may be used to represent logic ‘1’ and ‘0’ values with the option of connecting to neither when no data are to be transmitted. It should be appreciated by those skilled in the art that selector  901  may be configured to select between any number of current sources. Furthermore the inclusion of a not connected terminal is made by example only. Accordingly, selector  901  may modulate the IMON current output  111  to carry data to the control module. In some embodiments the modulation scheme may be such that the average current is not modified. The average current corresponds to the current monitor signal and its value is of importance. Accordingly, it may be preferred that the average current value remain unchanged. 
     Reference is made to  FIG. 10  which shows the embodiment of  FIG. 9  with the addition of low pass filter  1001 . Low pass filter  1001  is provided in the control module to extract the average current monitor signal when both data and the current monitor signal are transmitted at the same time from the TIA. Low pass filter  1001  receives the output of analogue to digital converter  807  and removes the high frequency components of the data. Accordingly, the control module is able to separate the current monitor signal and the data signal such that both may be transmitted concurrently. 
     It may be further desirable to be able to transmit data in both directions simultaneously while at the same time being able to observe the current monitor signal. Reference is made to  FIG. 11  which shows an embodiment comprising apparatus for transmitting data simultaneously in both directions between the controller and the TIA module without significantly affecting the current monitor signal. In  FIG. 11 , the IMON terminal is provided to the inverting input of operational amplifier  1101  in the control module. Feedback resistor  1103  is provided between the inverting input and the output of operational amplifier  1101 . The output of operational amplifier  1101  is further provided to filters  1105  and  1107 . Filter  1105  is configured to extract the average current signal and may be a low-pass filter. Filter  1107  is configured to extract the data from the output of the amplifier  1101  and may be a high-pass filter or a band-pass filter. A voltage selector  1109  is provided and is configured to provide one of a plurality of voltages to the non-inverting input of operational amplifier  1101 . Due to the high input impedance and high open-loop gain of an operational amplifier, the voltages at the inverting and non-inverting input of operational amplifier  1101  in normal operation are driven to be substantially the same. Accordingly, by modulating the voltage at the non-inverting input with voltage selector  1109 , the voltage at the IMON input is similarly modulated. In  FIG. 11 , voltage selector  1109  selects between three voltages. It should be appreciated that any number of voltages may be used. 
     By modifying the voltage in this way, the current monitor current may be modified in an unwanted way. Accordingly, it may be desirable to provide a compensation current to cancel out the current added to the current monitor signal. Resistor  1115  is provided between the inverting input of operational amplifier  1101  and voltage selector  1111  to provide a compensation current to the feedback loop of amplifier  1101 . Voltage selector  1111  is configured to be switched along with voltage selector  1109  such that the compensation current cancels out any voltage added to or subtracted from the output of amplifier  1101  as a result of modulating the voltage of the IMON line. 
       FIG. 12  shows an embodiment that does not require a compensation current. In the control module, the gate of a P-channel MOS (Metal Oxide Semiconductor) transistor  1203  is provided to the output of operational amplifier  1101  with the source provided to the non-inverting input. The drain of transistor  1101  is provided to filters  1105  and  1107  with a load resistor  1201  which converts the received current signal to a voltage. This arrangement is shown by way of example and other arrangements will be apparent to those skilled in the art. For example, in the case where the IMON average current is sunk by the TIA module the P-channel MOS transistor  1203  would be replaced by an N-channel MOS transistor and the resistor  1207  would be connected to a positive supply. It is also possible to use a bipolar transistor in place of the field effect transistor. Filters  1105  and  1107  may then separate the current monitor signal from the data. In this embodiment, voltage selector  1109  may modulate the voltage of the IMON signal without affecting the voltage observed across the load resistor  1201  and hence the average current and any data from the TIA module encoded in terms of current variations may be extracted without interference. 
     It is also noted herein that while the above describes embodiments of the invention, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention. 
     Some aspects and features of the disclosed embodiments are set out in the following numbered items: 
     1. A trans-impedance amplifier arrangement comprising: an input configured to receive an output from a photo-detector; a current monitoring circuit configured in use to provide a current monitor signal dependent on a current through said photo-detector; and an output configured to output said current monitor signal to a control module, said output further configured to receive control information from said control module to adjust the performance of said trans-impedance amplifier arrangement during the normal function of said arrangement for reception of optical signals carrying information. 
     2. The trans-impedance amplifier arrangement of item 1 wherein said output is further configured to provide said received control information to a control circuit. 
     3. The trans-impedance amplifier arrangement of item 2, wherein said control circuit is configured to compare at least part of said received control information with a predetermined code. 
     4. The trans-impedance amplifier arrangement of item 3, wherein said control circuit is configured to provide configuration information to one or more configurable modules of said trans-impedance amplifier arrangement in dependence on said the result of said comparing. 
     5. The trans-impedance amplifier arrangement of item 2, wherein said control circuit comprises a shift register for receiving said control information. 
     6. The trans-impedance amplifier arrangement of item 1, wherein said output is further configured to transmit information relating to at least one of configuration and internal circuit operating conditions of said trans-impedance amplifier arrangement to said control module, wherein said trans-impedance amplifier information is transmitted by said current monitor signal during the normal function of said trans-impedance amplifier arrangement for reception of optical signals carrying information. 
     7. The trans-impedance amplifier arrangement of item 6, comprising a selector configured to modulate said current monitor signal to transmit said trans-impedance amplifier operational information. 
     8. The trans-impedance amplifier arrangement of item 6, wherein one or more discrete current values are added at different times so as not to alter an average current value of said current monitor signal and to provide said trans-impedance amplifier operational information. 
     9. The trans-impedance amplifier arrangement of item 6 comprising: a first current source; a second current source; and a selector configured to connect the output to said first and second current sources dependent on said trans-impedance amplifier operational information. 
     10. A control module comprising: an input configured, in use, to receive data from a trans-impedance amplifier arrangement; and a control circuit configured, in use, to provide control information to said trans-impedance amplifier arrangement via an output, said control data configured to adjust the performance of said trans-impedance amplifier arrangement during the normal function of said trans-impedance amplifier arrangement for reception of optical signals carrying information. 
     11. The control module of item 10, wherein said control information comprises a control header. 
     12. The control module of item 11 wherein said control information comprises configuration information for one or more configurable modules of said trans-impedance amplifier arrangement to adjust the performance of said trans-impedance amplifier arrangement during the normal function of said arrangement for reception of optical signals carrying information. 
     13. The control module of item 10 wherein said control circuit is configured to transmit said control information with a modulated signal. 
     14. The control module of item 13, wherein one of a voltage and a current is modulated to provide said control information. 
     15. The control module of item 10, wherein said received current monitor signal comprises an average current signal and one or more discrete current values corresponding to trans-impedance amplifier information. 
     16. The control module of item 10, wherein said control module comprises a voltage selector configured to modulate a voltage of said output by modulating a reference voltage. 
     17. The control module of item 16, comprising an operational amplifier and wherein in use: an inverting input of said operational amplifier arrangement is provided to said output; and an non-inverting input of said operational amplifier arrangement is modulated in dependence on said control information. 
     18. The control module of item 17, wherein a compensation current is provided to said inverting input of said operational amplifier so as to substantially cancel out voltage changes at said output of said operational amplifier arrangement caused by modulating said non-inverting input. 
     19. The control module of item 16, comprising: a transistor configured to one of draw current from or supply current to said inverting input of said operational amplifier arrangement, said transistor being controlled by an output of said operational amplifier arrangement; and a load resistor configured to convert said current drawn or supplied by said transistor to a voltage. 
     20. The control module of item 15 comprising at least one of: a filter configured to extract said current monitor signal; and a filter configured to extract a signal corresponding to trans-impedance amplifier arrangement information.