A variable-gain current conveyor-based instrumentation amplifier without introducing distortion. An exemplary variable-gain instrumentation amplifier includes a first dual-output transconductance amplifier (DOTA) (i.e., current conveyor) that receives a first input voltage, a second DOTA that receives a second input voltage, a first resistive element connected between the first and second DOTA, an amplifier connected to the second DOTA at an inverting input, and a second resistive element that connects the second DOTA and the inverting input to an output of the amplifier. At least one of the resistive elements is a variable resistive element.

BACKGROUND OF THE INVENTION

CMOS transmission gate (T-gate) switches introduce distortion when used in an instrumentation amplifier (IA). A prior-art transmission gate circuit is shown inFIG. 1. Transistors MN1and MP1form the CMOS switch between pins “IN” and “OUT”. This switch is controlled by the “PGATE” and “NGATE” voltages, which are driven out of phase and rail to rail (0-5V) by two digital inverters.

The off-resistance of this switch is essentially infinite (>1000 meg). The on-resistance is typically between 50 and 5000 ohms, depending on design, and varies with process, supply voltage, and temperature (PVT). Furthermore, the on-resistance also varies with the voltage present at the “IN” and “OUT” terminals. The magnitude of this non-linear variation in resistance is typically significant, on the order of +/−10% of the nominal on-resistance over the full range of applied voltages in a given system. In amplifier circuits where low signal distortion is important, the effects of this non-linear on-resistance must always be mitigated.

SUMMARY OF THE INVENTION

The present invention provides a variable-gain current conveyor-based instrumentation amplifier without introducing transmission gate distortion.

An exemplary variable-gain instrumentation amplifier includes a first current conveyor that receives a first input voltage, a second current conveyor that receives a second input voltage, a first resistive element connected between the first and second current conveyors, an amplifier connected to the second current conveyor at an inverting input, and a second resistive element that connects the second current conveyor and the inverting input to an output of the amplifier. At least one of the resistive elements is a variable resistive element.

In one aspect of the invention, the current conveyors are dual-output trans conductance amplifiers (DOTAs).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2illustrates an instrumentation amplifier (IA)20. For clarity, dual-output transconductance amplifier (DOTA) symbols26,28are used to represent current conveyors. One of ordinary skill would understand how to apply current conveyors to the IA20. The gain of the IA20is defined by the following equation:
AV=VOUT/(V1-V2)=R2/R1(1)

As shown inFIG. 3, a IA40includes an additional resistor, R3, between resistor R1and the DOTA26. The voltage gain remains the same, Av=R2/R1, because there is no significant current flowing through the resistor R3, especially when using CMOS amplifiers, and no voltage is developed across the resistor R3.

Also in the IA40, a resistor R4is added between the resistor R1and the DOTA26. The gain remains the same for small values of the resistor R4. In this case, the same current that flows through the resistor R1also flows through the resistor R4but the voltage that develops across the resistor R4does not affect overall gain until the DOTA26saturates. For positive input voltages (V1-V2>0), this happens when the sum of the voltages across the resistors R4and R1is greater than the voltage across the resistor R2. Because the same current, I, flows through all three resistors, this is equivalent to saying that the sum of the resistors R4and R1must be less than the value of resistor R2to avoid amplifier saturation. In other words:
R4+R1<R2,  (2)
where it is assumed that all amplifiers are connected to the same power supply voltages, all amplifiers have the same saturation characteristics, and one of the inputs is at the ground potential (either V1=0 or V2=0, the worst case). This is equivalent to the condition:
R4<R2−R1(3)

Because Av=R2/R1, equation (3) is equivalent to equation (4) by substitution.
R4<R1(Av−1)  (4)

In most practical systems, the desired voltage gain is greater than unity and reasonable values of the resistor R4will not have any effect on the gain of the IA circuit40. In the same way, resistor R5does not affect IA voltage gain and small values of resistor R6(R6<R2−R1) do not affect gain, either.

In a similar manner,FIG. 4is a IA60that includes vestigial resistors R7and R8that are added to the negative input of the amplifier24without affecting the voltage gain of the IA60(Av=R2/R1). In the IA60, reasonable values of the resistor R7do not affect the gain because there is no current flowing through the resistor R7and the voltage across the resistor R7is insignificant (zero for CMOS amplifiers). However, if the value of the resistor R7is very large, it may affect high-frequency AC performance.

The gain of the IA60is also not affected by the value of the resistor R6, even though there is current flowing through the resistor R8, as long as the value is small enough to prevent saturation of the DOTA26output. Given the assumption that both amplifiers26and24are connected to the same power supply, have the same saturation characteristics, and that the node common to the resistors R2, R7, and R8is at zero volts (a virtual ground), then this condition is met when the voltage across the resistor R8is less than or equal to the voltage across the resistor R2. Because the same current, I, flows through both the resistors R2and R8, the gain of the IA60will not be affected by the value of the resistor R8, as long as the value of the resistor R8is less than or equal to the value of the resistor R2.
R8≦R2(5)

In one embodiment, as shown inFIG. 5-1, the gain of IA120is changed using a three-terminal potentiometer124. A potentiometer122is positioned between two DOTAs126,128that receive two different input voltages (V1, V2). The output of the second DOTA128is connected to the inverting input of a noninverting amplifier30. Functionally, all three circuits fromFIGS. 5-1through5-3have the following gain. Current conveyors are shown in U.S. Pat. Nos. 8,081,030 and 7,893,759, which are hereby incorporated by reference.
Av=R2/R1A(6)

The potentiometer122is modeled as a pair of resistors (R1A+R1B) such that the sum of the pair is a constant resistance (R1=R1A+R1B). When the wiper of the potentiometer122is at one extreme, the resistance between the wiper and the current-carrying end of the potentiometer122is a maximum of R1(R1A=R1and R1B=0). In this position, the gain of the IA120is equal to R2/R1. When the wiper is at the other extreme, where R1A=0 and R1B=R1, the gain of the IA is, in theory, R2/0 or infinity. As a practical matter, two effects will prevent the gain from actually going to infinity: (1) the open-loop gain of the input amplifiers and (2) the nonzero wiper contact resistance. Still, very high values of gain, on the order of 1000 or more are feasible.

As shown inFIG. 5-2, an IA140is configured similarly to the IA120, except that the potentiometer122receives the wiper from the second DOTA128.

As shown inFIG. 5-3, a IA150includes two potentiometers152,154, each having a maximum resistance of one half of the maximum resistance of the potentiometer122(FIGS. 5-1,5-2). The IA150has the advantage that the values of the noncurrent-carrying resistor segments ½R1Bmay be matched if the two potentiometers152,154are ganged together. In this case, whatever secondary effect these resistor segments have on the high-frequency AC response of the input amplifiers (i.e., DOTAs126,128) is equalized and the overall effect on bandwidth is minimized.

FIG. 6shows an IA170that includes an array172of six resistor segments that replace the resistor R1from the IAs shown inFIGS. 2 through 4. This resistor array172is connected to CMOS transmission gates SW1Athru SW3Bor some other active switch. The array172of resistor segments and digitally controlled T-gates is implemented on an integrated circuit; whereas the potentiometer approach is not. While this approach is not continuous and limits the IA gain to certain discrete steps, arrays of hundreds or thousands of resistor segments are feasible. In one embodiment, a digital logic block controls the CMOS transmission gates. The digital logic block may, in turn, be controlled by a microprocessor and computer program based on user input.

In the IA170there are three gain settings. Let A1denote the first gain setting (only switches SW1A, SW1Bare on) where A1=R2/Ri. Then, the second gain setting, with only the switches SW2A, SW2Bconducting, is A2=R2/R1/2=2 A1. In a similar manner, the third gain setting, with only the switches SW3A, SW3Bon, is A3=R2/R1/4=4 A1. To summarize, these three gain settings are related, as shown in Table 1. In one embodiment, the IA170does not require an array of equal-value resistor segments.

A significant consideration is the on-state resistance of the CMOS transmission gates172used as the switches shown inFIG. 6. The on-state resistance is not critical as long as it is less than R2−R1A. This requirement is most difficult to meet (smallest resistance) for the switches SW1A, SW1Band becomes progressively easier to meet for the switches SW2A, SW2Band then SW3A, SW3Bwhich benefit from progressively larger values of gain.

The transmission gates do not cause distortion in the IA170as long as the peak on-resistance is less than R2−R1. Transmission gates are not typically used in gain switching circuits where direct current flows through them because their on-resistance varies with the operating voltage.

As shown inFIG. 7, an IA200includes a common three-terminal potentiometer204for changing the gain of the circuit200. The potentiometer204is located between the second DOTA128and the amplifier130. The gain of the IA200is given by equation (7) below where a first resistor segment R2Aof the potentiometer204carries direct current and a second resistor segment R2Bdoes not carry direct current.
AV=R2A/R1(7)

In one embodiment, the potentiometer204is set so that the first resistor segment R2A=0 and the second resistor segment R2B=R2. Thus, the gain of the IA200is zero: Av=0/R1=0. At the other extreme, the potentiometer204is set so that first resistor segment R2A=R2(seeFIG. 2) and the second resistor segment R2B=0. Thus, the gain of the IA200is the same as the nominal gain using fixed resistors: Av=R2/R1. As a practical matter, the gain of the IA200cannot go to exactly zero, due to the finite terminal resistance of the potentiometer204. However, the gain may easily be reduced by three or four orders of magnitude from the nominal gain.

As shown inFIG. 8, an IA240includes an array244of three resistor segments and transmission gates, or other switches. Three possible gain settings are listed in Table 2 below where the nominal gain, A1, is R2/R1.

The IA240allows:

In one embodiment, the features shown inFIGS. 5 through 8may be combined to construct IA circuits whose gains may be varied over a very wide range of values: from zero to infinity with ideal components. Even with real components, the gain may be varied by at least six orders of magnitude. Furthermore, this may be accomplished with a high degree of gain accuracy and no amplifier distortion.