Wide Dynamic Range Current-to-Voltage Converters

Chunyan Wang

Department of Electrical and Computer EngineeringConcordia University

1455 de Maisonneuve Blvd. WestMontréal, Québec, Canada H3G 1M8E-mail: chunyan@ece.concordia.ca

Abstract - Current-to-voltage converters oftenneed to have a wide dynamic range and a highsensitivity. We propose a method to design suchconverters. The conversion is made to comprise anonlinear compression applied to the DCcomponent of the input current and a high gainconversion to the signal one. It is performed by alow-pass current filter and current-dependentresistances. The current filter is used to extractthe DC current from the input. This DC current isthen converted, with a compression, into a voltagebiasing the devices that make the signalconversion. In this way, the circuit bias and theconversion of the small signal are made input-current-adaptive. A CMOS converter has beendesigned with this method. The simulation resultsshow that is able to operate over a dynamic rangeof 4 decades.

I. INTRODUCTION

Current-to-voltage converters are important basicbuilding blocks in analog-mixed circuits. They arealso used in optical sensors to convert photocurrentsinto voltage signals. While the CMOS technology isevolving, the device feature size is shrinking and thesupply voltage is decreasing, which affects directlythe range of a voltage signal but not necessarily therange of a current. The dynamic range of a current-to-voltage converter is an important factor in itsquality metrics. One wishes to build a converter thatis able to respond to small signal variation whilecovering a wide range.

A converter of linear transconductance can notcover a wide range of signal variation, whereas thatof logarithmic character may not be able to have a

good sensitivity to detect small variations. Makingthe circuit characteristics adaptive to the inputcurrent is an effective approach to achieve a current-to-voltage conversion of wide-range and high-sensitivity. Delbruck’s adaptive sensor [1] has beenconsidered as a good example of this approach whilethe implementation of the adaptive element is acritically challenging task to make the sensoradaptive in a desirable way. The sensor circuitreported in [2] makes good use of the dependency ofthe MOS drain-source resistance on the current levelto make the “adaptability”. As the circuit involves ananalog switch, making the switching noise low is acrucial issue to secure a good operation.

In this paper, a method of developing wide-dynamic-range and high-sensitivity current-to-voltage converters is proposed. With this method, theDC and signal components of the input current areprocessed differently. Also, a design example usingthis method and the simulation results are presentedin the paper.

II. DESCRIPTION OF THE PROPOSED METHODAND ITS APPLICATION

The proposed method aims at solving the problem ofthe conflict of dynamic-range and sensitivity in thedesign and implementation of analog circuits, ingeneral, and current-to-voltage converters, inparticular. The input current of such a converter canbe expressed as iIN = IIN + iin, consisting of its DC

component and signal variation, and the outputvoltage is vOUT = VOUT + vout. It is usually case that

(a) iin, << IIN and (b) iin with a higher IIN is

statistically greater than iin with a lower IIN. In order

to make the converter circuit capable of detecting iin,

when IIN is very low and operating without saturation

when IIN is very high, it is desirable to have

VOUT = FCM (IIN) vout = rm(IIN) iin

where FCM (IIN) is a compression function of IIN and

rm(IIN) is a resistive coefficient of which the value

increases if IIN decreases. In this way, the DC

component of iIN is to be compressed, which can be

logarithmically, while the signal iin is converted to

vout with a gain adapting to its current level. The

implementation of this principle consists of (i)separating IIN and iin, (ii) implementing FCM (IIN) and

(iii) implementing rm (IIN).

A separation of IIN from iIN can be done by a low-

pass filter for a current signal. Such a current filtercan be easily built based on a current mirror, as shownin Fig. 1, in which the time constant τ = C/gm1, with

gm1 being the transconductance of N1, is greater than

1/fmin, where fmin is the lowest frequency component

of the signal iin. Under this condition, the common

gate voltage Vgn is not able to respond to the varying

iin and thus the currents in the two NMOS transistors

can follow IIN only. If the transistors are matched, the

drain voltage of the NMOS in the right side should beequal to Vgn. Thus, the DC component IIN is

converted, via Vgn, into the voltage VOUT with a

nonlinear compression. If the current is weak enoughto drive the NMOS transistors in the weak inversionmode, the compression will be logarithmic.

It should be noted that, in the circuit shown inFig. 1, if the current injected into the drain of N2

deviates slightly from IIN, the drain voltage will

respond by a deviation from VOUT. The deviation is

related to the finite drain-source resistance resultingfrom the channel length modulation [2].

Using the principles described above, we proposea basic circuit structure, as shown in Fig. 2, for thewide-dynamic and high-sensitivity current-to-voltageconversion. In this circuit, two copies of the inputcurrent iIN are made by means of the PMOS

transistors P1 and P2. The first copy is used to extract

the DC component IIN, of which the mirrored copy is

shown as IIN’, taking a slight device mismatch into

consideration. The second one, illustrated in Fig. 2 asiIN’, is combined with IIN’ at the node vOUT to get iinthat produces vout = rm(IIN) iin, where rm(IIN), the

gain for the signal variation, is determined byrdsP2//rdsN2, the drain-source resistance of P2 in

parallel with rdsN2, that of N2. The gain rm(IIN) is

current-dependent and its value decreases when IIN

increases, which makes the circuit to have a highersensitivity to an input signal at a lower level.However, if IIN is constant and N2 and P2 are in the

saturation mode, the gain is constant to make a linearconversion from iin, to vout.

Fig. 1. Current-mirror-based low-pass filter for current sig-nal. If the time constant t = C/gm1 is large enough, only IIN

will be mirrored to the right side of the circuit.

Fig. 2. Basic structure of the proposed current-to-voltageconverter. The current filter shown in Fig. 1 is incorporatedto make IIN’, a copy of the DC component of the input cur-rent iIN. The output voltage vOUT is modulated by vout =rm(IIN) iin, where rm(IIN) = rdsP2 //rdsN2.

iIN = IIN + iin

IIN

C

Vgn

N1 N2

gm1

vOUT

iIN IIN’

C

Vgn

Vgp iIN’

vOUT

N1 N2

P0 P1 P2

To apply the proposed method in a design of acurrent-to-voltage converter of wide-dynamic-rangeand high-sensitivity, a good low-pass current filteringand an adaptive gain rm(IIN) are the two key issues. In

the circuit shown in Fig. 2, while the adaptive gain isimplemented easily by using the channel-lengthmodulation effect, the low-pass filtering is done underthe condition that τ = C/gm1 > 1/fmin, in which gm1 is

determined by the current in the transistor N1. To

make the condition τ > 1/fmin, satisfied over a very

wide current range, gm1 needs to be somehow “input-

current-insensitive”. It can be done by making thecurrent in N1 to vary in a much smaller scale than that

of the input current. One may add a current branch inparallel with the transistor P0 to divide iIN into two

portions. The portion taken by the added branch ismade to increase with iIN in such a way that the

portion taken by P0 is “stabilized” to avoid a drastic

increase of gm1 when iIN becomes very large. One can

use one PMOS and one NMOS in series to make sucha branch. The PMOS has its source and gate incommon with P0 and its drain connected to the input

current source via the NMOS, of which the gate iscontrolled by Vgn to make the conductivity of this

branch to depend on iIN.

Also, to satisfy the filtering condition, one canplace a large capacitance at the common gate of theNMOS transistors N1 and N2, which, however, may

make Vgn to respond too slowly to a change of IIN.

One solution to this problem is to add a NMOStransistor connecting VDD and Vgn nodes. Its gate

should be controlled by vOUT so that if Vgn is not able

to respond fast enough to an increase of IIN, vOUT will

be high to drive the NMOS on, adding a current toaccelerate the Vgn updating.

III. SIMULATION RESULTS:

The circuit in Fig. 2 is able to operate over a inputcurrent signal (iin) range from 1 nA to 1 μA, which

has been confirmed by a HSPICE simulation with thetransistor models of a 0.18 μm CMOS technology.On the basis of this circuit, the branch for the currentdivision and the NMOS transistor to accelerate the

Vgn change have been added. With these two

additions, the higher end of the iin range is extended

to 10 μA, and the simulation waveforms when theinput variation is of 1 MHz are illustrated in Fig. 3. Ithas been observed that the circuit provides a non-linear compression applied to the DC component and,at a given level of IIN, approximately linear

conversion for the current variation. It should bementioned, however, that the conversion gain vout/iinis variable depending on the input current level IIN. It

is about 105 mV/μA when iin= 1 nA and IIN = 10 nA,

and 20 mV/μA when the input current iIN is 10,000

times stronger. Thus, the output voltage is able torespond to the input current variation over the entireinput current range.

Fig. 3. Simulation waveforms. The input current iIN is log-arithmically scaled and the output voltage vOUT linearlyscaled. The frequency of the current signal iin is 1 MHz, andthe ratio of iin/IIN is 1/10.

IV. CONCLUSION

We have proposed a method of designing current-to-voltage converters aiming at a wide dynamic rangeof the input current and a high sensitivity to smallsignal variations. The conversion makes good use of alow-pass current filter to extract the signal componentof the input current, and a variable resistance that can

be a drain-source resistance of a MOSFET to convertthe current variation into a voltage signal in anadaptive manner. With this method, current-to-voltage converters can be made to have a nonlinearcompression applied to the DC component of theinput, a very high gain to small current signal and alower gain if the signal level is higher. Unlike manycurrent-mode circuits, the converters do not needclock and switches, and the conversion is performedon a continuous time basis. Thus the problems suchas switching noise and preparation phases do notarise. Employing this method, we have designed aCMOS current-to-voltage convert and the simulationresults show that the circuit is able to operate over asignal current range of four decades. It is suitable forapplications in optical sensors to deal with a verywide dynamic range of incident signal without losingsensitivity to weak signals. The circuit can be

implemented in a standard CMOS process withoutany need for linear devices.

ACKNOWLEDGMENTS

The author thanks Natural Science andEngineering Council of Canada for the financialsupport and CMC Microsystems for CAD resources.

REFERENCES

[1] T. Delbruck., and C. Mead, “Adaptive photoreceptorwith wide dynamic range,” Proc. IEEE InternationalSymposium on Circuits and Systems, May 1994, vol.4pp. 339 - 342.

[2] C. Wang and F. Devos, “An Adaptive optical sensor,”Proc. IEEE International Symposium on Circuits andSystems, Geneva, Switzerland, May, 2000, vol.4 pp.333-336.