用商业实验室设备测量超低偏置电流的提示|电子设计

This article is part ofTechXchange：Why Low Iq is the Smart Thing to Do

What you'll learn:

How to effectively measure ultra-low bias current by recreating the measurement scheme from an app note.
测量输入电容和输入电流。
Creating a better measurement environment.

In applications that require low leakage current, it’s important to select a low-input-bias-current (I_b) operational amplifier. The application noteAN-1373describes how to measure ultra-low bias current using theADA4530-1evaluation board. However, due to the nature of handling femtoampere-level currents, the measurement environment—equipment such as jigs, shields, cables, and connectors—also affect the measurement results.

本文将介绍一项试验，以使用常用的商业级实验室设备，夹具和材料来重新创建AN-1373的测量，并包括一些解决方法以改进测量，以最终达到50 FA。

首先,我们测量偏差的输入电容current and the variation of output voltage with charging of the input capacitance under the condition of 125°C. We also attempt to derive the bias-current value from the measured output voltage. Finally, we will try to improve the measurement environment based on the measurement results.

电容性集成测量

根据AN-1373，输入电容（C_p必须先测量ADA4530-1的），以便使用电容积分测量方法。我们将使用ADA4530-1R-EBZ-BUF,with the ADA4530-1 configured in buffer mode.

Next, we calculate the input current (I_B+). Specifically, using the circuit configuration shown in数字1，我_B+流入C_p当测试框中的SW从ON（接地到GND）转到OFF（打开）时。当我时，输出电压上升_B+费用c_p, so that the value of I_B+can be calculated by monitoring and substituting it into Equation 1.

用输入串联电阻测量总输入电容

计算c_p, this experiment adopts a method using series resistance.数字2显示了一个简单的电路图。的价值eries resistance is based on the measurement guidelines found on page 6 of AN-1373. The actual value is R_s= 8.68 MΩ. An SW also is mounted in the test box for later experiments (SW is open at this time).

The frequency at which the waveform from the function generator is attenuated to –3 dB can be measured; use Equation 2 to calculate the input capacitance.

数字3shows the setup. Since the temperature in the temperature-controlled chamber rises to 125°C in the experiment described in the section “Measuring I_B+with Known Input Capacitance” (page 6 of AN-1373), we utilize materials that can withstand such a temperature. RG-316U was used as the material for the coaxial cable.

Furthermore, the noninverting inputs of the ADA4530-1 on the evaluation board are triaxial connectors. For this reason, a triax-to-coaxial conversion connector (BJ-TXP-1 from the Axis Company) was employed. In this configuration, the guard terminal on the triax side was left floating.

As a result of the measurement, C_p= 73.6 pF，这是一个相对较大的值，因为根据AN-1373，实际测量值约为2 pf。其原因与测试框的电缆长度有关（看起来更像是测试板）到非注射输入。

FORTE – The intelligent BOM tool

用更大的智能掌握您的材料清单。

Learn More

Measuring I_B+with Known Input Capacitance

最后，我们开始测量偏置电流。电路配置显示在数字1, and数字4illustrates the mounted test box. Note that the input resistor used in the section “Measuring Total Input Capacitive with an Input Series Resistor” is removed.

如AN-1373中所述（电容性集成测量方法，第7页），短路SW到GND，然后将其打开并监视数字万用表（DMM）的输出电压波动几分钟（我们使用了34401AKeysight Technologies的DMM）。最后，计算i_B+通过替换v_出去into Equation 1.

The results of three measurements under the same conditions are revealed in数字5。图的下部显示了由DMM测量的ADA4530-1的输出电压波动，并且上部显示了使用等式1计算的当前值。对于所有三个实例，在测得的电压值中都没有可重复性。因此，计算出的电流值的波形也与AN-1373中描述的结果不同（请参阅AN-1373中的图13和14）。

How to Improve the Measurement Environment

在“电容性集成测量”部分中，我们测量了I_B+基于AN-1373，但结果有所不同。在本节中，我们共享改善测量环境的步骤，从而获得测量的准确性。

安装屏蔽盒并缩短输入电缆

首先，我们进行了以下两个改进：

A shield box was installed on the evaluation board inside the thermostatic chamber(Fig. 6)。
缩短了连接到非注射输入端子的同轴电缆以减少C_p(Fig. 7)。

For one, we expect to reduce the effect of external noise, and secondly, we expect to reduce the small leakage current in the cable (the recalculated C_p是35.2 pf）。但是，尽管采取并重新测量了这些措施，但未观察到可重复性，类似于“电容性整合测量”中获得的结果。波形与预期的显着不同。

删除测试框

取出了测试盒，通过直接短路和打开地面更改了SW(Fig. 8)。In other words, the conductance component called the test box was removed and the measurement was performed. As a result, we were able to obtain the waveform as shown in数字9。

DMM测量的输出电压随着恒定的斜率增加，并在所有测量值中达到4.16 V左右。相应的电流显示约50 fa的值。

Furthermore, the red line in数字9shows the waveform of the re-measurement with a shorter coaxial cable connected to the noninverting input terminal (C_p= 26.5 pf）。电压上升的斜率与理论计算一样大。从这些测量结果中可以发现，输入侧的电导成分对测量精度具有显着不利影响。