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degisn & prdoucts Anaglo degisn Avoiding amplifier output driver saturation By Jon Munson and Kevin Scott 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 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 the LTC6268 500MHz femptoamp 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 = 0. In actual practice, a zero bias current amplifier is unrealistic. However, the LTC6268’s ±3fA typical bias current and ±4pA overtemperature bias current sets the standard for wide bandwidth, Fig. 1: IBIAS error in photodiode signal conditioning application Fig. 2: LTC6091: Saturated output may cause input common mode violation 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 LTC6091 buffer circuit in Figure 2 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, 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. 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 amplifiers with a wide range of performance specifications and power supply voltage requirements. These devices can help you realize your sensor design at optimum performance levels. Jon Munson is Applications Engineer at Linear Technology Corp – www.linear.com Kevin Scott is Strategic Marketing Engineer at Linear Technology Corp. Fig. 3: LTC6091 50pA IBIAS amplifier input stage Fig. 4: 10kΩ resistors prevent amplifier output from saturating 36 Electronic Engineering Times Europe January 2016 www.electronics-eetimes.com


EETE JAN 2016
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