When taking sensor measurements, the type of sensor excitation used typically 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 is often 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 pictorially in Figure 1, where a 500MHz femptoamp (fA) bias current FET-input amplifier is used to convert photocurrent into a voltage measurement.
Ideally the photodiode current (IPD) would equal the feedback current (IFB) and IBIAS would equal zero.
In practice, a zero bias current amplifier is unrealistic.
When output saturation occurs
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 buffer circuit shown in Figure 2 using an LTC6091 is an example where this can occur. The LTC6091 is a dual, 140V precision amplifier with a 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, as shown in Figure 3.
Amplifier input structure
The input stage consists of +INA and -INA, which are the gates of the amplifier’s 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 down the input 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 indeterminate output state. If the indeterminate state leads to the output remaining saturated, then additional bias current is required to restore normal operation.
Figure 4 shows a simple solution to the problem in an instrumentation amplifier circuit using a three amplifier configuration.
The design uses a single amplifier version of the dual LTC6091, and the LT5400-2 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).
The conclusion is that low bias current amplifiers with the right specifications and power supply voltage requirements can help you in your sensor sub-system design.
Jon Munson, applications engineer and Kevin Scott, strategic marketing engineer, Linear Technology