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Overvoltage protection for sensitive amplifier applications–Part 2

(This is Part 2 of the feature. Click here to see Part 1.)

Diode protection to ground

Shunting overvoltage current to the supply rails means the power supplies must absorb the current. Many power supplies cannot sink current. Shunting overvoltage current to a supply that cannot sink current is acceptable if the total load connected to the power supply is larger than the fault current, or if overvoltage protection exists at the supply. If nothing within the circuit can sink the overvoltage current, the supply voltage will rise, causing potential damage to components connected to the supply.


Figure 2: Zener Diode Protection to Ground
(Click to Enlarge Image)

A Zener diode can be used as an overvoltage protector that shunts fault current through ground via a current-limiting resistor (Figure 2 ). Note that the Zener protection is effective only if the Zener voltage is less than the supply voltage.

The overvoltage current is limited by RLIMIT according to the following equation:

where VFBZ is the forward drop of the Zener diode and VRBZ is the reverse drop of the Zener diode, both of which depend on temperature and bias current. The sum of the reverse and forward drops must be less than the supply rails so that the internal ESD diodes of the amplifier do not turn on.

In general, Zener diodes exhibit reverse-leakage currents that are higher than regular silicon diodes. This reverse current increases dramatically near the break-over voltage and, in the I vs. V curve, is sometimes referred to as the “knee”. If the input signal swing is large, non-linearities are introduced. A Zener diode's capacitance also varies considerably with applied voltage and is typically much higher than a normal silicon diode.

You can improve the bandwidth and leakage characteristics of the Zener protection method by connecting Zeners in parallel and adding normal silicon diodes in series (Figure 3 ). The overvoltage current is limited by RLIMIT according to the following equation:


Figure 3: Improved Zener Diode Protection to Ground
(Click to Enlarge Image)

The result is a reduction in total capacitance such that the input signal source roughly sees only 2 * CR . This technique also reduces the leakage to roughly that of normal silicon diodes. Note that all the above diode protection methods work in the inverting op-amp configuration as well.

Differential-diode protection One of the best methods to keep leakage current and capacitance constant is to keep the voltage across the protection diodes equal to 0 V. The differential diode protection method keeps a 0 V bias across the protection diodes during normal amplifier operation (Figure 4 ). During an overvoltage, the diodes conduct fault current to ground.


Figure 4: Differential Diode Protection
(Click to Enlarge Image)

For the inverting op-amp configuration the overvoltage current is limited by RLIMIT according to the following equation:

For the non-inverting op-amp configuration, the overvoltage current is limited by RLIMIT according to the following equation:

Signal-protector integrated circuits Signal protector ICs provide overvoltage detection circuitry in combination with MOSFET switches (Figure 5).


Figure 5: Signal Protector Method
(Click to Enlarge Image)

While the input signal is within the supply rails, the signal protector acts as a series resistor. When an overvoltage condition occurs, the signal protector acts as an open circuit.

There are several possible advantages to using a signal protector. First, the leakage currents are small enough for many applications (±500 pA max at 25° C for the MAX4505). Second, there isn't the strong relationship between input voltage and leakage current or capacitance that is inherent to passive devices. Third, when the supplies are off, the signal protector can handle ±40 V at its input without any damage, while maintaining its output at 0 V.

Unfortunately, the fault recovery time may be too slow for some applications. In addition, if cost is an important factor, the discrete solutions may be more suitable.

Noise considerations

Amplifier bias currents contain noise. When this current noise flows through a resistor, a voltage noise is produced. In addition to producing noise due to amplifier bias current, a resistor produces thermal noise that is equal to the square root of 4kTBR, where k is Boltzmann's constant, T is the temperature in Kelvin, B is the bandwidth, and R is the resistance.

The total input referred noise of an operational amplifier circuit is given by the following equation:

(Click to Enlarge Image)

Rp and Rn are the resistances looking out of the op-amp positive and negative inputs. Rn is normally equal to the gain setting resistors in parallel (RF //RI ). Vp and Vn are the voltage noises of the op-amp positive and negative inputs. Ip and In are the current noises of the op-amp positive and negative inputs.

From the equation above, you can see that RLIMIT will contribute to the system noise by contributing to Rp or Rn (depending on configuration). If you use Zener overvoltage protection, make sure to add the noise of the Zener into the equation.

References
1. Op-Amp Applications Handbook (by Walt Jung, Analog Devices)
2. Overvoltage Effects on Analog Integrated Circuits (by Adolfo Garcia, Wes Freeman, Analog Devices)
3. Analysis and Design of Analog Integrated Circuits (by Gray, Hurst, Lewis and Meyer)
4. Intuitive IC Op Amps (Thomas M. Frederiksen)
5. Noise Reduction Techniques in Electronic Systems (Henry W. Ott)

About the authors

Erik Anderson and Eric Schlaepfer are with Maxim Integrated Products, Sunnyvale, CA.

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