Avoid Amplifier Output Driver Saturation When Using pA Bias Current Amplifiers with High Source Impedance Sensors

Need for Low Bias Current Amplfiers

When taking sensor measurements, the type of sensor excitation used varies greatly; it can be a DC signal, an AC signal, a voltage source, a current source or a pulsed source to name a few. When using current source excitation or when using a high impedance sensor, the amplifier's bias current often is an important specification, as it can create an undesirable voltage error term as the bias current flows through an external resistance. For this reason, low bias current amplifiers are often required in many of these applications.

This is shown pictorally in Figure 1, where the LTC6268 500MHz fempto-amp bias current FET-input amplifier is used to convert photocurrent into a voltage measurement. Ideally the photodiode current (I_PD) would equal the feedback current (I_FB) and I_BIAS would be = 0. In actual practice, a zero bias current amplifier is unrealistic. However, the LTC6268's ±3fA typical bias current and ±4pA over temperature bias current sets the standard for wide bandwidth, low bias current amplifiers.

Output Saturation

Sensors that require low bias current amplifiers include photodiodes, accelerometers, chemical sensors, piezoelectric or piezoresistive pressure transducers and hydrophones. Using a low bias current amplifier with a high impedance sensor can cause problems if the amplifier's input is overdriven, which can lead to an increase in bias current. When this occurs, the amplifier may get "stuck", with the input signal no longer capable of pulling down the output signal to remedy the condition. The following LTC6091 buffer circuit is an example where this can easily occur. The LTC6091 is a dual, 140V precision amplifier with only 50pA bias current (max at 25°C), a rail-to-rail output swing and only 50µV of input offset voltage. Its common mode range is limited to 3V from the power supply rails

To understand what is happening, let's first look at the input stage of the amplifier.

Amplifier Input Structure

The input stage consists of +INA and -INA, which are the gates of the amplifiers first stage N-MOSFET differential pair. When the output saturates due to an input overdrive, there needs to be bias current through the input protection network to pull the input down sufficiently so the device can come out of saturation. However, the high source impedance is unable to furnish much bias current to begin with, and once the input is overdriven and the output saturates, the -INA input can be pulled up so that it now exceeds the common mode voltage range. In this situation, ,the differential pair can shut off, resulting in an indetermnate output state. If the indeterminate state leads to the output remaining saturated, then additional bias current is required to restore normal operation.

The Solution

Figure 4 shows a simple solution to the problem in an instrumentation amplifier circuit using a three amplifier configuration. The LTC6090 is a single amplifier version of the dual LTC6091, and the LT5400-2 is a quad matched resistor network with ±75V operation, four 100kΩ resistors and better than 0.01% resistor matching.

Two 10kΩ resistors are added at the output to limit the worst-case output swing and prevent the feedback voltage from ever exceeding the input common mode range of the amplifier. Empirical tests show that above 20MΩ source resistance, there may not be adequate bias current to "free" the +INA input if it were to get "stuck". With lower source resistances, it is possible to pull down the +INA input after an overdrive event against the protection device leakage current that must be overcome (i.e. the high-Z input source retains control).

Linear Technology offers a wide array of low bias current amplfiers with a wide range of performance specifications and power supply voltage requirements. These devices can help you realize your sensor design with the performance you have come to expect from our products. Contact your local sales office for additional assistance.

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