A voltage biased superconducting quantum interference device bootstrap circuit

Abstract

We present a dc superconducting quantum interference device (SQUID) readout circuit operating in the voltage bias mode and called a SQUID bootstrap circuit (SBC). The SBC is an alternative implementation of two existing methods for suppression of room-temperature amplifier noise: additional voltage feedback and current feedback. Two circuit branches are connected in parallel. In the dc SQUID branch, an inductively coupled coil connected in series provides the bias current feedback for enhancing the flux-to-current coefficient. The circuit branch parallel to the dc SQUID branch contains an inductively coupled voltage feedback coil with a shunt resistor in series for suppressing the preamplifier noise current by increasing the dynamic resistance. We show that the SBC effectively reduces the preamplifier noise to below the SQUID intrinsic noise. For a helium-cooled planar SQUID magnetometer with a SQUID inductance of 350 pH, a flux noise of about 3 μ 0 Hz −1/2 and a magnetic field resolution of less than 3 fT Hz −1/2 were obtained. The SBC leads to a convenient direct readout electronics for a dc SQUID with a wider adjustment tolerance than other feedback schemes.

Practical dc SQUID system: Devices and electronics

“…We can define a dimensionless parameter n = M i  (@I/@U) to quantitatively describe the strength of CFC [4]. At n = 0, there is no current feedback.…”

“…In Ref. [4], this is called the ''critical condition'' because the feedback completely cancels the current. In the case of n > 1, hysteresis appears in the IÀU characteristics, thus leading to a system instability [9].…”

“…Generally, the system noise depends on SQUID intrinsic noise, on the SQUID's flux-to-voltage transfer coefficient @V/@U, and on the preamplifier noise, which includes two sources: a voltage source V n and a current source I n . For both voltage-and current-bias mode, different voltage feedback circuits (VFC), such as additional positive feedback (APF) [2], noise cancelation (NC) [3] and SQUID bootstrap circuit (SBC) [4], have been introduced to increase @V/@U. Another way is to employ a weakly damped SQUID with large StewartMcCumber parameter b c , which exhibits a large @V/@U of several hundred lV/U 0 [5].…”

An insight into voltage-biased superconducting quantum interference devices

“…The flux-to-field coefficient @B/@U of the magnetometers was measured to be 1.5 nT/U 0 . Two planar coils, L 1 and L 2 , which can be used to implement suitable current and voltage feedbacks of SQUID bootstrap circuitry (SBC), 7 were also integrated in the design as an option. The coil L FLL was integrated on the chip for the operation in fluxlocked loop mode.…”

“…We then used the additional feedback circuitry, SBC, 7 which is already integrated on our SQUID chip, to further reduce ͱS U . Using the SBC readout scheme, the flux noise of SQUID (II) reached about 3 lU 0 /ͱHz (ͱS B < 5 fT/ͱHz), as shown in Fig.…”