FEATURES

• High transimpedance gain
• Wide intrinsic bandwidth
• Low noise
• Low offset
• Excellent linearity
• Adjustable bandpass filter
• Dc offset adjustment
• Screened amplifier circuitry
• Low cost

APPLICATIONS

• Medical instruments
• Metrology
• Laboratory instrumentation
• Detection system development
• Remote sensing

DESCRIPTION

The BD101 device is a low noise pre-amplifier module for current output sensors. The module is ideally suited to conditioning the signals from photodiodes and similar devices. As such it is an excellent development tool. User selectable gain, frequency bandwidth selection and offset adjustments are all provided in one compact package.
Great care has been taken to ensure the best possible signal to noise ratio. This is reflected in the co-operative interaction of the bandpass and gain selection which in response to the user inputs balances the permutations of the amplifier gain and filter stages to optimise the signal to noise ratio.

The input signal is coupled via a standard BNC connector on the left hand side of the front panel. The adjacent two controls determine the filter characteristics of the amplifier. The bandwidth is from true DC to 100KHz (3dB). The high frequency response can be reduced in decade steps down to 1Hz. If DC performance is not required the low frequency response can be increased in decade steps up to 10KHz. The transimpedance gain (the input current is scaled to a proportional voltage) can be varied from 10KW to 100MW in decade steps. If the DC component of the input signal needs to be compensated a offset adjustment feature is provided.

The offset voltage is derived from a highly stable bandgap reference and is continuously adjustable from -1.2V to +1.2V.The output signal is available for further processing via the BNC connector on the right side of the front panel. All sensitive amplifier circuitry is screened in a metal enclosure to provide the highest possible immunity from radiated interference. The only additional components required for operation of the device are the sensor element, a low noise stabilised power supply +5 to 15 V and an oscilloscope or similar suitable equipment to view the output.

CIRCUIT OPERATION

To get the maximum benefit from the use of the pre-amplifier module some rudimentary knowledge of the circuit functions is required. To illustrate the operation of the circuitry, a photodiode sensor is used. Photodiodes are typical current output sensors.

A photodiode produces a photocurrent in response to the incident light power. In fact, the output current typically varies linearly with incident optical power over seven decades. To convert the photocurrent to a voltage for subsequent processing a classical transimpedance amplifier is used. The favoured circuit configurations using operational amplifiers are depicted below.

The first circuit produces the lowest offset and is preferred for high sensitivity measurements that require operation down to DC levels. The second circuit reverse biases the photodiode which can dramatically increase the speed of response of the complete circuit (this will be explained later). Hence, this second configuration is preferred when speed of response is more important than DC performance.

To understand the circuitry of the pre-amplifier module the simple approximations shown above must be considered in more detail. In both circuit variants the photodiode is depicted as an ideal circuit element. As a current source it should have infinite output resistance and for maximum speed of response to varying input signals it should have zero capacitance. Obviously, this is not the case in real life. All photodiodes have a capacitive element that increases with increasing active area. Hence, large area photodiodes should not be used to detect high frequency, low level optical signals (large capacitance also increases the noise of the circuit combination). The resistance of photodiodes is usually high (megaohms at least) but it is finite and these characteristics impose limitations on the conditioning circuitry. Consider the real world scenario depicted below for the photovoltaic mode.

This realistic scenario includes the model of the photodiode and a more realistic ac model of the amplifier which will introduce additional noise into the measurement process. The amplifier noise performance is specified by two parameters. The input noise voltage, En, is specified related to the bandwidth of the amplifier combination. For the pre-amplifier module the bandwidth is set by the bandpass filter lowpass setting. For a gain setting of 1MW or less the typical noise voltage is:

If the contribution from the current noise is ignored the output from the first stage of the pre-amplifier becomes:

As can be seen from the above, the noise voltage is multiplied by a frequency dependent component. If the amplifier gain response is considered as below.

From the above the noise gain will start to increase at the frequency determined by the values of the photodiode junction capacitance, Cj, and the amplifier transimpedance gain (Rf). The noise gain will continue to increase until the feedback capacitance (Cf) limits the gain. The above illustrates the fundamental conflict between resolution (detecting smaller amounts of light) and speed (higher frequencies). The limiting factor in this conflict is the amount of noise gain permitted. The pre-amplifier module is designed such that as the gain is increased the feedback capacitance is automatically adjusted to ensure a consistent bandwidth response. The user must decide the level of gain the signal from the sensor can tolerate before other subsequent noise reduction techniques.

To achieve transimpedance gains higher than 1MW , additional gain stages are used in the pre-amplifier module. This ensures the bandwidth of the pre-amplifier can be usefully increased without a corresponding increase in noise. Balancing of the gain between the first and subsequent stages is achieved automatically inside the pre-amplifier module.

The above has not considered the affects of unwanted DC signals. There are a couple of amplifier induced signals that will introduce a DC error. If the bandpass filter highpass selection is not set to DC operation these signals will not be relevant. DC operation is not important solely for slowly varying input signals. Consider the scenario where the photodiode is being used to detect pulses of light of varying magnitude. If the Bandpass filter is not selected to operate down to DC levels then if a large pulse signal is followed by a much smaller pulse signal, the following could occur.

If the large pulse signal precedes a much smaller faster signal then the capacitive coupling is still recovering to the base level when the signal is detected and the smaller signal is easily lost in this region. This scenario is often the case when simple level threshold is subsequently used.

The two parameters of the amplifier that affect the DC performance are the Input offset voltage (Vio) and the Input bias currents (Ib). The offset voltage produces a DC offset that has a temperature dependence. For pre-amplifier gains of 1MW or less the worst case first stage offset at 25° C is 0.1mV. With an additional worst case 1.5mV from subsequent gain stages. For gains above 1MW , the offset increases proportional to the additional gain. As the temperature of the first stage amplifier increases, the first stage offset increases with a worst case figure of 1mV/°C. The offset from the latter gain stages increases by a worst case 20mV/°C.

The bias current that flows into the input of the amplifier will produce an offset due to the current flow through the feedback resistor. Hence, the total offset from the bias currents is Ib.Gain. For a maximum 100MW gain the worst case offset will be 4mV at 25° C. Unfortunately, it is a fundamental property of amplifiers that the bias current increases with temperature.

It should be borne in mind that the bias current and hence, offset produced, will double every 8° C increase in temperature.

Because of the above an offset null feature is built into the pre-amplifier module. This uses a extremely stable reference voltage of 1.2V. The offset applied to the output signal can be continuously varied from +1.2V to -1.2V. This feature permits the use of additional gain stages subsequent to the pre-amplifier module to enable the resolution of extreme low level signals.

In addition, the photodiode used will have a leakage current that will increase with increasing bias voltage. For the zero bias circuit, the bias signal across the photodiode is the voltage offset between the amplifier terminals. For the pre-amplifier model this is typically 0.1mV at 25° C. Leakage current of the photodiode is related to the active area of the device and other device processing parameters. A small photodiode (e.g. 1mm² active area) will have shunt resistance in excess of 100MW . This will generate a leakage current when biased of 1pA. However, for large active area devices (e.g. 100mm² active area) this leakage current could be 20pA. At this magnitude the photodiode leakage would introduce a significant additional offset determined by the leakage current.Gain.

If the reverse bias circuit configuration is used the photodiode junction capacitance, Cj will reduce exponentially. This is illustrated below.

Note if the reverse bias configuration is to be used it is advisable to keep the bias voltage within the pre-amplifier module supply voltage or to include additional protection components. Otherwise, a photodiode breakdown could damage the pre-amplifier module input circuitry. The reduction in photodiode junction capacitance reduces the high frequency noise gain enabling higher speed operation.

However, the reverse bias greatly increases the photodiode leakage current. At 12V reverse bias, a typical 1mm2 photodiode would have a typical leakage current of 200pA. At a gain setting of 100MW this would introduce an additional offset at the amplifier output of 20mV!

Whilst every effort has been made in the design of the pre-amplifier module to limit the unwanted influence of the component circuitry on the signal measurement. In many cases the greatest unwanted signals will be due to external interference. To minimise the affect of this noise source, all of the amplifier circuitry is housed in a screened enclosure. Routing of the input and output signals are via screened coaxial cables. It is strongly recommended that this procedure is adopted by the user when connecting the input and output signals.

Finally, one component outside the influence of the pre-amplifier module is the power supply. The module will operate with power supplies from ±5V to ±15V. Use of a low noise, regulated power supply is essential if high sensitivity measurements are to be possible. Bear in mind that above 1KHz the ability of the amplifier stages to reject power supply noise diminishes.