Selecting the right input module

The Model 9210 comes with three different input module options, 9210-LO, 9210-MED and 9210-HI. This note focuses on the difference between the different options.

Background

The input noise, input impedance, input bias- and input offset current are depending on each other, and when selecting the right input option, the most critical parameter needs to be identified depending on the application. In general, a low input noise density comes along with a low input impedance and high input bias current while low input offset/bias currents and a high input impedance is accompanied by a higher input noise density.

The following table gives an overview of the different parameters.

Comparison of typical input module properties
Module Input noise Input impedance Input bias current Input offset current Noise equivalent resistance High-gain bandwidth
9210-LO 1.8 nV/√Hz ~1 GΩ 15 nA 0.5 nA ~200 Ω 100 kHz
9210-MED 4 nV/√Hz ~30 GΩ 0.1 nA 0.1 nA ~1 kΩ 100 kHz
9210-HI 18 nV/√Hz ~1 TΩ 10 pA 0.6 pA ~20 kΩ 10 kHz

Thermal noise

Thermal (Johnson/Nyquist) noise is arising from any resistor due to thermal fluctuation of the charge carriers. Connecting a resistor, probe or sample having a source resistance R to the amplifier input will result in an additional noise besides the amplifiers own input noise. At a given temperature T, thermal noise density vn specified in V/√Hz is given by
    vn = √(4kBTR),
where kB is the Boltzmann constant. Therefore, for high source resistances, the noise floor is not dominated by the input amplifier noise but the source resistance, and a lower amplifier noise does not improve the measurement result. The above table lists the source resistance where the thermal noise is similar to the different modules noise floor.

Input bias/offset current

Due to the non-ideal behavior of amplifiers, a small leakage current is always flowing through all inputs of an amplifier. This current is called input bias current, while the difference in current between the two inputs is called input offset current. These currents are dependent on both temperature and common mode voltage, and can differ between different amplifiers. The most noticeable side effects can be self-heating of the sample and a DC offset. The lock-in signal itself is usually not affected by these currents.

Input Impedance/capacitance

At higher frequencies, cable capacitance can have a strong effect on the measurement results. Coaxial cables and Ethernet cables typically have a capacitance in the order of 50 pF per Meter, resulting in a cable impedance of XC = 1/(2πfC). Normally, effects from cable capacitance are much larger then the amplifiers own input capacitance.

Dynamic Reserve

The dynamic reserve is the maximum ratio between the smallest detectable signal and  a disturbing signal at a different frequency.

Range 9210-LO 9210-MED 9210-HI
10 V >140 dB >140 dB >140 dB
5 V      
2 V      
1 V      
500 mV      
250 mV      
10 0mV      
50 mV      
20 mV      
10 mV      
4 mV      
2 mV 120 dB 113 dB 100 dB

Summary

Application 9210-LO 9210-MED 9210-Hi
Superconductors ++ + -
Semiconductors + ++ -
Thermometers -- 0 ++