USB Isolator

Avoid interference and earth loops

Published in issue 10/2012 on page 14

If your USB device ever suffers from noise caused by an earth loop or if you want to protect your PC against external voltages then you need a USB isolator. The circuit described here offers an optimal electrical isolation of both the data lines as well as the supply lines between the PC and the USB device.The project itself is a spin-off from an earlier design for a portable oscilloscope for Android.

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Products

Components

Resistors
(1% 0.1W, SMD 0603, unless otherwise shown)
R1 = 1kOhm 1% 0.5W (SMD 1206)
R2 = 33kOhm
R3 = 80.6kOhm
R4 = 6.04kOhm
R5 = 200kOhm
R6 = 90.9kOhm
R7,R8 = 28.7kOhm
R9,R10,R11,R12 = 24Ohm
R13,R14 = 4.7kOhm
Capacitors
C1,C2 = 10nF 16V X7R (SMD 0603)
C3 = 220nF 50V X5R (SMD 0603)
C4,C5,C6 = 120µF 6.3V (Nichicon RFS0J121MCN1GS, type C)
C7,C8,C9,C10,C11,C12 = 100nF 25V X7R (SMD 0603)
C13,C14,C15,C16 = 10µF 6.3V X5R (SMD 0805)
Inductors
L1,L2,L3,L4,L5,L6,L7,L8 = ferrite inductor 33Ohm@100MHz (SMD 0603) e.g. Murata BLM18PG330SN1D
L9,L10 = 1µH 1.2A (SMD 1007) e.g. Taiyo Yuden CB2518T1R0M
Semiconductors
D1 = PMEG3050BEP (NXP) (SOD-128)
D2 = PMEG6010CEH (NXP) (SOD-123f)
D3,D4 = 5.6V 3W zener diode
IC1 = LT3575EFE#PBF (LT) (TTSOP-16)
IC2 = ADUM3160BRWZ (AD) (SOICW-16)
IC3 = NCP380HSN10AAT1G (On Semiconductor) (TSOP-5)
IC4,IC5,IC6 = NCP380HSN05AAT1G (On Semiconductor) (TSOP-5)
IC7,IC8 = TPD2EUSB30ADRTR (TI) (SOT-3)
Miscellaneous
S1,S2 = slide switch with changeover contact (e.g. C&K Components PCM12SMTR)
K1,K2 = USB mini-B-connector, PCB mount (e.g. On-Shore USB-M26FTR)
K3 = DC adapter connector, 1.35 mm (e.g. CUI PJ-014DH-SMT)
K4 = USB-A connector, PCB mount (e.g. FCI 87583-2010BLF)
TR1 = transformer 3:1, 25 µH (e.g. Würth 750310471)
Case, Hammond type 1593KBK
2 self tapping screws, cross head, 4x6.4mm, DIN7981
PCB # 120291-1 or
assembled and tested board and case, # 120291-91