Reproduced Blog: Are you violating your op amp’s input common-mode range?

You went through a process to select an operational amplifier (op amp) for your circuit based on the parameters most critical to your application. Some of the parameters you reviewed may have included supply voltage, gain bandwidth product, slew rate, and input noise voltage, to name a few.

You also accounted for input common-mode range, a key parameter important for all op amp applications in your circuit, right? If your answer is no, then you are highly encouraged to continue reading this article. Even if your answer is yes, you may still find this material useful.

Engineers who have worked with op amps throughout their careers likely have experienced situations where an op amp was behaving in an unexpected manner. The nice thing about op amps is that the output often tells the story. In many cases, if something is not “quite right”, it shows up in an obvious way at the output pin. Undesirable output waveforms can be caused by limitations at the output stage. Perhaps an oscillation is observed that is caused by too much capacitance on the output. Or maybe clipping occurs before reaching the full rail voltage because the output stage is limited to voltage swings less than the supply rail voltage.

It is also possible for strange behavior to appear at the op amp’s output that has nothing to do with the output stage. Sometimes the undesirable output signal may result from something wrong at the input side of the device. One of the most common issues experienced with op amps is violation of the device’s input common-mode range. But what exactly is input common-mode range, and what is the impact of violating or exceeding it?

Defining input common-mode range

When speaking of op amp inputs, input common-mode voltage (VICM) is one of the first terms of which an engineer thinks, but may lead to some initial confusion. VICM describes a particular voltage level and is defined as the average voltage at the inverting and non-inverting input pins (Figure 1).

Figure 1: Input common-mode voltage for an op amp.

It is commonly expressed as:

VICM = [VIN (+) + VIN (–)]/2.

Another way to think of VICM is that it is the voltage level common to both non-inverting and inverting inputs, VIN (+) and VIN (–). As it turns out, in most applications VIN (+) is very close to VIN (–) because closed-loop negative feedback causes one input pin to closely track the other such that the difference between VIN (+) and VIN (–) is close to zero. T

This is true for many common circuits, including voltage followers, inverting, and non-inverting configurations. In these cases it is commonly assumed that VIN (+) = VIN (–) = VICM, since these voltages are approximately the same.

Another term used to describe op amp inputs is input common-mode range (VICMR), or more correctly input common-mode voltage range. This is the parameter most often used in datasheets and is also the one where circuit designers should be most concerned. VICMRdefines a range of common-mode input voltages that results in proper operation of the op amp device, and describes how close the inputs can get to either supply rail.

Another way to think of VICMR is that it describes a range defined by VICMR_MIN and VICMR_MAX. As shown in Figure 2, VICMR is described by:

VICMR = VICMR_MAX – VICMR_MIN

Where:

VICMR_MIN = limit relative to VCC – supply rail

VICMR_MAX = limit relative to VCC+ supply rail

Figure 2: Input common-mode voltage range for op amp.

When VICMR is exceeded, the normal linear operation of the op amp is not guaranteed. Therefore, it is critical to ensure that the entire range of the input signal is fully understood and that VICMR is not exceeded.

Another point of confusion may be that VICM and VICMR are not standardized abbreviations, and various datasheets from various IC suppliers often use different terminology including VCM, VIC, VCMR, etc. Consequently, it is necessary to understand that the specification you’re looking for is more than a particular input voltage – it is an input voltage range.

VICMR varies among op amps

The input stage of an op amp is dictated by design specifications and the type of op amp process technology used. For example, the input stage of a CMOS op amp is different than that of a bipolar op amp, which is different than that of a JFET op amp, etc. While the specific details of op amp input stages and process technologies are beyond the scope of this article, it is important to note these differences exist among various op amp devices.

Table 1 shows several examples of op amps from Texas Instruments (TI) and their VICMR. The Max Supply Range column describes split-supply and single-supply (in parentheses) limitations. From the table it is clear that the input range, VICMR, is quite different from op amp to op amp. Depending on the type of device, VICMR may fall within or beyond the supply rails. Hence, never assume that an op amp can receive a particular input signal range until it is verified in the datasheet specifications.

 

Table 1: VICMR examples for several different types of op amps.

One special case worth mentioning for wide input ranges is the rail-to-rail input op amp. Although the name implies an op amp whose input can span the entire supply rail range, not all rail-to-rail input devices cover the entire supply range as many might assume. It’s true that many rail-to-rail input op amps do span the entire supply range (such as the OPA333 in Table 1), but there are others that fall a little short and are misleading in their description. Again, it is critical to review the specified input range in the datasheet.

Examples of violating VICMR

Violating VICMR is commonly seen in single-supply op amp applications where the negative rail is often ground, or 0V, and the positive rail is some positive voltage such as 3.3V, 5V, or higher voltages. In these applications the input signal range typically is not very wide, and the input signal and VICMR must be well understood to make sure proper op amp operation results. If VICMR is violated, undesirable output behavior can result such as clipping the signal at voltage levels lower than expected, voltage shifts in the output signal, phase reversal, or the output reaches one of the supply rail voltages prematurely.

To better understand the effects of exceeding VICMR, we created some examples with violations. We selected two op amps with different VICMR specifications to demonstrate these effects. We chose these devices because they have rail-to-rail outputs to rule out limitations due to the output stage. The single-supply voltage follower circuit in Figure 3 was used to capture waveforms for both devices. All data was captured on a lab bench at ~25°C room temperature.

 

 

Figure 3: Single-supply voltage follower circuit used for evaluating VICMR.

Example 1

For the first example, we chose a TLC2272 op amp and supplied it with VCC = 10V. The datasheet describes its typical VICMR range as –0.3 to 4.2V for a 5V supply voltage at 25°C. Note the input limitation near the positive supply rail, at .8V below VCC (or VCC –.8V). In this example VCC = 10V is used and the resulting input limit near VCC is estimated to be about 9.2V.

To test the circuit, we apply a 300 Hz sine wave with DC offset of VCC/2 = 5V to the input. The AC amplitude is adjusted until a change is observed at VOUT. As shown in Figure 4, when 10 Vp-p input is applied the VOUT shows a clipped signal near the positive rail, but not near the negative rail. This undesirable behavior near the positive rail is what we should expect, if the input exceeds VCC – 0.8V, or in this case 9.2V. For VIN levels below 9.2V and down to 0V, VOUTshows a proper waveform, as expected.

 Figure 4: VOUT of TLC2272 shows clipping when VIN (Ch1) exceeds 9.2V.

Example 2

In the second example, a TL971 rail-to-rail output op amp is used in the Figure 3 voltage follower circuit, but with different results. Here, the op amp is supplied with a 5V single supply, such that VCC = 5V. From the datasheet specifications, the guaranteed VICMR range spans 1.15V to 3.85V, or roughly 2.7 Vp-p centered at VCC/2. A 1-kHz sine wave is applied with a DC offset of 2.5V. The VIN amplitude is adjusted from 200 mVp-p to larger levels until a change is observed at VOUT.

With VIN centered at VCC/2 = 2.5V, VIN is increased to 2.7 Vp-p with expected linear behavior at VOUT. As VIN is increased up to about 3.5 Vp-p (centered at 2.5V), VOUT continues to follow VINand exhibits proper op amp behavior. Note that the linear behavior is better than what we might expect from the datasheet limits for VICMR, but it still exceeds the guaranteed limits.

As VIN is increased slightly more to 3.52 Vp-p, VOUT starts to exhibit non-linear behavior near both the positive (5V) and negative (0V) rails (Figure 5). VIN is further increased to 4.2 Vp-p to clearly exceed VICMR. As the input peak exceeds the limit near the positive rail, the signal at VOUT rails out as it jumps up to the positive rail (5V) and stays there until VIN returns to an acceptable range (Figure 6). As the input drops below the limit near the negative rail, the signal at VOUT exhibits a phase-reversal as it jumps to mid-rail (2.5V) and tracks VIN with an offset until VIN increases to an acceptable voltage within the VICMR.


Figure 5: Onset of non-linear output behavior for TL971 when VIN = 3.52 Vp-p.


Figure 6: Non-linear output behavior for TL971 when VIN = 4.2 Vp-p.

These examples show that different non-linear behavior can result from different types of op amps when VICMR is exceeded. Even though phase reversal resulted in the second case, note that phase-reversals do not occur in every op amp when VICMR is violated – it just depends on the op amp.

DC analysis

In the previous examples, we used an AC signal to evaluate VICMR for an op amp circuit. Another useful test is to apply a DC voltage source to the input of the circuit in Figure 3. While varying the DC input, the output level behaves in a similar manner, except that it won’t be varying over time. Depending on the type of circuit, AC or DC analysis (or both) may be useful in the early evaluation of the op amp.

Overcoming a VICMR problem

What if you discover that you can’t meet the VICMR requirements of your op amp late in the design process? Maybe the other device parameters are ideal for your application, and it is really difficult to change the device. One or more of the following options might be a potential solution:

(a)    If the input amplitude is too large, use a resistor divider to keep the signal within proper range of VICMR.

(b)   If the input signal offset is the problem, try using an input biasing or DC offset circuit to place the input signal within the specified VICMR range for the op amp.

(c)    Change the device to a rail-to-rail input op amp that meets all your other requirements.

References

  • Download datasheets for op amps used in these examples here: OPA333, TL971,TLC2272.
  • Download your free version of TINA-TI™, a SPICE-based analog simulation program used in these examples: www.ti.com/tinati-ca.

Reproduced Planet Analog Blog: Are you violating your op amp’s input common-mode range? written by Todd Toporski

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