An integrated digital SQUID magnetometer with high sensitivity input
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2142 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5, NO. 2, JUNE 1995
An Integrated Digital SQUID Magnetometer with High Sensitivity Input
Masoud Radparvar and Sergey Rylov HYPRES, Inc., 175 Clearbrook Rd., Elmsford, NY 10523
Abstract-A single chip SQUID magnetometer is described that integrates a SQUID-based pre-amplifier with a h i sensitivity comparator gate and feedback circuitries on the same chip. The compprstorgate is an asymmetric SQUID gate drMng two SQUID quantizem in series with the feedlwclt coil. The chip's sensitivity and noise level are primarily determined by the pre-amplifier SQUID. The pick up coil is in series with the feedback transformer. Since the current in the feedback coil is maintained close to zero, the dynamic range of the chip cpll be extremely wide and is independent of the SQUID p"pMi'ir orcmqmrafor architectures. The chip's slew rate Ss determined by the bipolar clock biasing the comparator gate. Clocks running in the tens of MIEZ result in a magnetometer systemwith slew rate exceeding le @As (ao = 2.07Xl(r7 Gauss-cm~. This chip simplifies room temperature electronics and, due to its digital output, can be easily multiplexed on-chip. A system based on this chip can be operated in a relatively h i magnetic field environment without extensive magnetic shielding. The details of the chip as well as preliminary measurement results for the pre- amplifier ps well as the digital circuit will be presented.
In order to utilize a SQUID as an amplifier, its periodic transfer characteristic should be linearized by a feedback loop with high open loop gains. This linearization has the added benefit of substantially increasing the dynamic range of the SQUID circuit. The function of the feedback coil is to produce a field which is equal but of opposite polarity to the applied field. To simplify the sophisticated peripheral electronics, various types of digital and single chip SQUID magnetometers have been proposed and demonstrated[ 1-81. Many of these magnetometers either suffer i?om low dynamic range and/or poor energy sensitivity. The small dynamic range is due to the size of the on-chip feedback coil that can only accommodate a limited number of fluxons circulating in the loop. In aprevious publication, we demonstrated a single- chip SQUID magnetometer with practically unlimited dynamic range. This was achieved by combining the input and the feedback signals in the same superconducting loop, thus keeping the current in this feedback loop close to zero regardless of the magnitude of the input signal. However, the input SQUID inductance was limited to 1.5 pH, thus limiting the energy Sensitivity of the resulting magnetometer chip. To the best of our knowledge, none of the other single-chip magnetometers have dem-ated energy sensitivity suitable for practical applications. In this paper, we review the design of a high sensitivity single- chip magnetometer that combines the previously developed single-chip magnetometer with a SQUID pre-amplifier. Preliminary measurement results for the pre-amplifier as well as the digital SQUm circuit will be presented.
Manuscript received Oct. 18,1994.
II. CIRCUIT ARCHITECTURE
In order for single-chip (digital) SQUID magnetometers to be o f d a l value, they must, at least, have an energy sensitivity
puts the stringent criteria on the components of the single-chip magnetometer. The pre-amplifier determines the energy sensitivity. However, the compa"s current (or magnetic field) sensitivity should be also adequate to be able to exploit the pre- amplifier's sensitivity. This should be accomplished without sacrificing the margins on the bias c m t of the winparator. To meet these objatives, the SQUID pre-amplifier can be coupled to a DC array SQUID amplifier before integrating it to the comparator SQUID. To improve the sensitivity of the comparator, it may be designed to have a multi-loop SQUID washer to allow coupling ofmulti-turn coils to it. Figure 1 shows the circuit diagram for such a high sensitivity single chip SQUID magnetometer. It consists of an analog SQUID, a DC SQUID amy atnpl ik , a comparator gate and a feedback circuit with two write gates
spproachingtheir CCMII&-P& analog SQUIDS. This requirement
All currents in m.4 Allrabtorr.hOh
x 100 0.75 m0.7S
i=O.3 CLOCKBIAS COMPARATOR SQUID
Fig 1 Circuit d h p m for a very sensitive singlechip SQUID magnetometer with DC SQUID array pre-amplifier and feedback Circuitry. The comparator
. t h e
limit tothe dynamic range forthis type of SQUIDmagnetometer since the input flux is cancelled by the feedback aurent. Dynsmic range is only
of the input analog SQUID. The pre-amplifier d i t was developed and & " h i a t NIST, and fabricated and demonstrated at HYPRES. The pic* up coil is integrated with the feedback loop to improve the sensitivity
consists of 8 washers to improve its sensitivity by coupling of large inductance to its loop indud" . Thereisnointrkic
limited bythe Current-carryingcapacity ofthe input coil and the sensitivity
ofthe over-all chip. couprig coefficient between inductors is assumedto
1051-8223/95$04.00 0 1995 IEEE
The operation of the pre-amplifier circuit has been discussed in detail by Welty and Martinis in a separate paper[lO]. In this ckuit, an input signal couples flux into the input SQUID, which is voltage-biased with a 0.05 R resistor, so that the SQUID current is modulated by variations in the applied flux. The flux modulation coil of the output SQUID is connected in series with the input SQUID, so that variation in the input SQUID current changes the flux applied to the output array. The series array is biased at a constant current, so the output voltage is modulated by this applied flux. The extemal field sensed by the hgh sensitivity analog SQUID is converted to a current that is applied, through the series array of inductors, to the series DC SQUIDS. These DC SQUIDs have current-voltage characteristics of shunted Josephson junctions, but about 100 times larger dynamic resistance. The series array of DC SQUIDs can generate a DC voltage on the order of milli-volts, that can be even directly ufilized by room temperature electronics and feedback circuitry to read out and apply to the high sensitivity analog SQUID.
The operation of the single-chip magnetometer has been described in a separate paper and briefly, is as follows. The comparator SQUID has an asymmetric threshold characteristic and is biased slightly over its critical current using a bipolar current source. In the absence of any external field, the output voltage is also a bipolar voltage. For a sufficiently large applied magnetic field, the comparator only generates pulses in response to either positive or the negative portion of the applied gate current depending on its polarity. The write gates are designed to have asymmetric threshold characteristics and are biased below their thresholds. The bipolar current induced in the control lines of the write gates will cause the right and the left write gates to cross their lobes only in the positive and negative directions, respectively and produce SFQ pulses upon lobe crossing. When the comparator SQUID pulses negatively, the left write gates launches fluxons into the storage loop. (Alternately, when the comparator pulses positively, the other write gate launches antifluxons into the storage loop.) The injection of only fluxons or only antifluxons would continue in each clock period as long as the gate current of the comparator is below comparator's threshold current for positive or negative currents. With proper polarity, the SFQ-induced current in the superconducting feedback loop can eventually cancel the applied current and restore the comparator SQUID close to its original state. When the current in the feedback loop is close to zero, both write gates, altemately, emit fluxons and antitluxons into the loop in each clock period, keeping the feedback current close to zero. One advantage of this scheme is that the size of the feedback loop can be very small and is actually determined by the desired signal slew rate and SQUID sensitivity. The polarity of the missing pulses determines the direction of the applied field, and the switching probability leads to a voltage across the comparator SQUID which is a measure of the strength of the input signal. The digital output is the difference between the number of negative and positive pulses across the comparator. A multi-bit up/down counter coupled to the output of the one-bit comparator can count the down pulses and subtract these from the up pulses to exhibit the output in digital form.
111. CIRCUIT DESIGN
A typical analog SQUID possesses a flux sensitivity better than 6x1 O-' (PJHz" which is more than adequate for many practical applications. Due to the complexity of single-chip magnetometers and the ease of multiplexing, these chips are best suited for multi- channel biomedical systems, such as encephalograms. Such systems require a field sensitivity of 10 ff /Hz" for an input coil of around 1 pH and pick up coil area of A=l cm2. This gives rise to a needed current sensitivity of 1 pA/Hz". Required dynamic range is determined by interference which has a slew rate of approximately 3 pT/s at 60 Hz for a gradiometer system in a typical environment. The field sensitivity of the input pre- amplifier SQUID (B,) should be better than 10 ff/Hz", or B, = a, n /A c 10 f f k " , where a, is the flux noise of the SQUID and n is the current transfer ratio between the transformer and the SQUID loop inductance (LJ. Assuming #,= 6x107 a&", then an upper bound for the turn ratio is about 830 turns for a pick up coilareaof1cm2. SinceL,-L, -