Lake Shore Model 370 AC Resistance Bridge User’s Manual
Scanners for the Model 370
Three custom scanners are available with the Model 370, the Model 3716, 3716L, and the 3708. These are designed
specifically to increase the Model 370 input capability from 1 to either 8 or up to 16 resistors without sacrificing
measurement performance.
Each scanner is housed in a separate enclosure and can be mounted directly on the cryostat so leads carrying the most
sensitive low voltage signals are as short as possible. The scanner also allows extension of the Model 370 shield to all
resistor leads. A preamplifier in the scanner amplifies measurement signals before they travel through the longer leads to the
Model 370. Different preamplifiers in the 3716, 3716L, and 3708 optimize them for different applications. The Model 370
supplies power and control to the scanner, eliminating additional noise from AC power lines, ground loops, and computer
interface connections.
Scanner operation is fully integrated so most of the Model 370 hardware and firmware features can be used with the scanner.
Supported hardware features include matched impedance current source and ground isolation. The scanners provide up to
four 25-pin D-sub connectors for resistance inputs. Each connector accommodates four inputs, with four signal and two
shield lines for each input. Guarding is supported between the instrument and scanner only. Interface cables from the Model
370 to the scanner are included with the scanner.
Supported firmware features include temperature conversion, math functions, linear equations, and in some applications,
temperature control. The Model 370 can store measurement range and temperature conversion data for each resistor.
Appropriate parameter values are automatically recalled with input changes, reducing interface overhead and settling time.
As with any scanner, the resistance reading rate is slower during input changes, resulting in longer filter settling times and a
longer sample period for temperature control. The Model 370 can be used with third party scanners; however, access to
integrated features is lost.
The Model 3716 Scanner
The Model 3716 mirrors the single input of the Model 370 that is optimized for low residual DC bias current. Low bias
provides the lowest available resistor self-heating when excitation currents are in the range of 1 pA to 30 pA. It also provides
the best available accuracy when resistances are above 200 k
Ω
. The tradeoff for these performance features is a slightly
greater noise figure (33 nV
⁄
√
Hz) than the Models 3716L and 3708. Unused leads are connected to measurement common to
reduce noise pickup, a persistent problem when measuring large resistances. Performance of the Model 3716 scanner is so
nearly identical to the Model 370 that they share specifications for resistance range, accuracy, and resolution (noise).
The Model 3716L Low Resistance Scanner
The Model 3716L is optimized for low input noise figure (4 nV/
√
Hz) and can achieve measurement resolution to
20 n
Ω
. The scanner’s lower noise figure actually improves the measurement resolution of a standalone Model 370 over much
of its range. It is the best choice for general measurement applications that do not require excitation current below 100 pA
and resistance above 100 k
Ω
. The 3716L also enables some measurements, such as low resistance Hall effect measurements,
that would not be possible with the 3716. Unused leads are left open, allowing the scanner to perform Hall effect
measurement sequencing. Please note that the specifications for resistance range, accuracy, and resolution (noise) are
different than the standalone Model 370.
The Model 3708 Ultra-Low Resistance Preamp/Scanner
For ultra-low AC resistance measurement applications that demand the very best in low noise performance, the Model 3708
8-channel preamp/scanner is the best choice. At just 2 nVRMS/
√
Hz, the Model 3708 offers the lowest input voltage noise of
the three available scanners and can achieve measurement resolution to 10 n
Ω
. The Model 3708 improves low ohmic and low
resistance Hall effect measurement capability by a factor of two over the Model 3716L. Like the Model 3716L, unused leads
are left open to facilitate Hall effect measurement sequencing. With DC bias current of 55 pA, however, it is not
recommended for subkelvin temperature measurements. These measurements require very low DC bias current to prevent
measurement errors as a result of self heating. Specifications for resistance range, accuracy, and resolution (noise) are
different than the standalone Model 370.
1-4
Introduction
