High CMRR and Wideband Current Feedback Instrumentation Amplifier Using Current Conveyors

This paper presents a high CMRR and wideband current feedback Instrumentation Amplifier (IA). The proposed IA architecture consists of Fully Balanced Differential Difference Amplifier (FBDDA) and Differential Difference Amplifier (DDA) based on 2 generation current conveyor (CCII) with a buffer. From the simulation results evaluated by HSPICE, the proposed IA exhibits average CMRR was 109.3 dB higher than the conventional one. Furthermore, the proposed IA has higher closed-loop gain over a larger bandwidth than corresponding voltage feedback.


Introduction
Instrumentation Amplifier (IA) with high CMRR, high input impedance, and configurable differential gain characteristics is used in many application areas, such as medical instrumentation, the read-out circuit of biosensors, signal processing, and data acquisition [1].
Fig. 1 shows the conventional IA, which consists of 3 operational amplifiers and 7 resistors.Each resistors require the condition of R1 = R2, R3 = R4, R5 = R6.In this case, the output voltage is described by Eq. ( 1).However, CMRR, especially common-mode gain (Ac) of this IA is highly dependent on strict resistor matching [2].For example, if only 0.1% mismatch between R5 / R3 and R6 / R4, CMRR deteriorates from ideal (infinity) to 66 dB, on condition that differential gain is 0 dB [5].Thus, resistor mismatches are serious problem for IA.
To overcome this problem, we have proposed an IA architecture which consists of Fully Balanced Differential Difference Amplifier (FBDDA) and Differential Difference Amplifier (DDA) [4].Its CMRR is extremely higher than that of the conventional IA under the condition of resistor mismatch [3].
On the other hand, in a signal processing system, one of the considerable characteristics is the bandwidth.

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Especially, in the biological signal processing, the chopper stabilization technique is often employed for avoiding 1/f noise in MOS devices [4].In this kind of signal processing, wide bandwidth circuits are required strongly.However, the conventional IA exhibits a narrow bandwidth that is highly dependent on the closed-loop gain, due to the fixed gain bandwidth product.
In this paper, high CMRR and wideband IA, which consists of FBDDA and DDA using current feedback technique, is presented.The proposed IA has advantages that common-mode gain is insensitive to the resistor mismatches and bandwidth is wider than the conventional one and is independent of gain.The proposed IA is evaluated through HSPICE with the set of the parameters of 0.6m CMOS process.In this paper, we report the detailed simulation results.

Voltage Feedback and Current Feedback
Voltage feedback is majority feedback method for the conventional IA.Fig. 2 shows the block diagram of voltage feedback.A(s) is the open-loop gain as a function of frequency, β is a feedback factor which is amount of feedback from output.Fig. 3. represents frequency response of Fig. 2. Fig. 3 indicates that if we get higher closed-loop gain reducing β, closed-loop bandwidth will be narrow caused by constant Gain Bandwidth Product (GBP).Therefore, IA with voltage feedback is not suitable for implementation of IA with the chopper stabilization technique and high gain.
Fig. 4 shows the block diagram of current feedback.Z(S) is the open-loop trans-impedance gain as a function of frequency, gin is a trans-conductance which converts input voltage to current, gf is a trans-conductance which feedback output signal as current.Fig. 5 denotes typical frequency response of Fig. 4. As long as gf is fixed, the bandwidth is constant, even if the closed-loop gain is varied by changing gin [6].Therefore, current feedback has wider bandwidth than voltage feedback, and its bandwidth is independent of the closed-loop gain.Therefore, the current feedback is suitable for high gain IA with the chopper stabilization technique.terminal Yi (i=1, 2, 3) exhibits an infinite input impedance in ideal, the voltage at Xi (i=1, 2, 3) follows that applied to Yi, X exhibits a zero input impedance in ideal.The current supplied to X is conveyed to the high impedance output terminal Zi [7].Theoretically, each resistors of the proposed IA no require resistor matches for high CMRR.Furthermore, the proposed IA has higher closed-loop gain over a larger bandwidth than corresponding conventional IA.

Proposed Instrumentation Amplifier
The output voltage of 1 st stage (VA and VB) is given as follows in the case of R1 = R2.
From Eq. ( 2) and ( 3), common-mode signal can be rejected at VA and VB before 2 nd stage.In addition, output voltage of 2 nd stage is given by Then, from Eq. ( 2), ( 3) and ( 4), we can derive Vout as follows.
In mismatch condition (R1 ≠ R2), Vout,m can be given by Eq. ( 6) represents common mode signal is rejected even if the mismatches in all resistors are occurred.Thus, the proposed IA has much higher CMRR than the conventional one.

Simulation Results
In this chapter, simulation results of CMRR and frequency response are shown.The conventional IA and proposed IA were evaluated using HSPICE with 1P 3M 0.6μm CMOS process.

CMRR
For comparison of conventional and proposed IA, we have simulated 300 times with ±30% all resistor mismatches by Monte Carlo analysis.The simulation result is shown in Fig. 7. To evaluate the effect of AC which is more sensitive to resitor mismatch than differential gain (AD), AD of both IAs are set 0 dB for easy estimation.
From Fig. 7, average CMRR of conventional and proposed IA are 24.9 dB and 134.2 dB respectively.The average CMRR of the proposed IA is 109.3dB higher than conventional one.

Frequency Response
Fig. 8 and Fig. 9 show the frequency response of the conventional IA and proposed IA, respectively under the condition that the differential gain of both IAs are varied from 12 dB to 20 dB by changing RG.As closed-loop gain of the conventional IA is getting higher, the cut-off frequency is getting lower.On the other hand, the cut-off frequency of the proposed IA is constant, regardless of change of the closed-loop gain.Therefore, proposed IA is superior to conventional IA in terms of bandwidth.
Lastly, the detailed simulation results are listed in Table 1.From these results, the performance of the proposed IA is much better than that of conventional one.

Conclusion
In this paper, high CMRR and wideband current feedback instrumentation amplifier using CCII have been presented.The CMRR and frequency response of conventional and proposed IA were evaluated by HSPICE.As a result, we confirmed the proposed IA has 109.3dB higher CMRR than the conventional one under the resistor mismatches.Moreover, it was verified proposed IA has higher closed-loop gain over a larger bandwidth than corresponding conventional IA.
The evaluation through the actual fabrication of the LSI is the future work.

Fig. 6 Fig. 4 .Fig. 5 .
Fig. 6 shows the proposed architecture for the high CMRR and wideband IA.The proposed IA consists of 1 st

Table 1 .
Summary of the Simulation Results.