Open-loop test of input offset voltage

Input offset voltage (VIO) is an important electrical performance parameter of voltage comparator (hereinafter referred to as comparator). GB/T 6798-1996 defines it as "the DC compensation voltage added between the two input terminals when the output voltage is a specified value". Most traditional test equipment adopts the test mode of "Device Under Test (DUT) - auxiliary operational amplifier". The test schematic diagram is shown in Figure 1.

Under the action of auxiliary amplifier A, the whole system forms a stable closed-loop network, so that VD=0, then
VC = -VS1·R1／R2

In this way, the output of the DUT can be controlled by adjusting the external power supply VS1. When VC is equal to the specified voltage,
VIO = VA - VB
is obviously < and VB = 0

By measuring the output voltage VE of the auxiliary operational amplifier A, VIO can be calculated.

In the above closed loop, the working state of DUT is no different from that of an ordinary operational amplifier. The advantage of this test is that the output of DUT can be conveniently clamped at the specified value through the external power supply VS1. At the same time, since VIO is mostly in the millivolt level, VIO is amplified to the volt level for testing, which does not require high test equipment, but is greatly affected by interference signals.

The IC test equipment ASL-1000 developed by Credence in the United States is configured with two DVI_300s, one ACS, one TMU, one DOAL, and one MUX. The difference in output characteristics between the comparator and the operational amplifier and the circuit characteristics of the operational amplifier test loop DOAL (Dual Op Amp Loop) determine that the comparator test on ASL-1000 cannot form a closed-loop network using DOAL to achieve DUT output clamping like the test of the operational amplifier. Therefore, two Force and Measure sources dvi_9 and dvi_11 are used, supplemented by some resources of doal_8, to form an open-loop test circuit for VIO. Figure 2 is the test schematic diagram of LM311 (single comparator) VIO.

Unlike a closed-loop network, such an open-loop test circuit cannot clamp the output voltage Vout of the DUT at any specified value. Therefore, a successive approximation test method is adopted. dvi_9_channel_0 and dvi_11_channel_0 are used to provide working power to the DUT. The former also supplies power to the pull-up resistor R1. Obviously, if R1 is not provided, the Vout of the DUT cannot approach the specified 1.4V. dvi_11_channel_1 applies a voltage to the same-direction end of the DUT, with an initial value of 12mV. Since the reverse end is grounded, under normal circumstances, the dvi_9_channel_1 measurement value of Vout measured at the output end of the DUT should be greater than 1.4V, and then the voltage at the same-direction end is reduced in a certain step. Once Vout jumps from greater than 1.4V to less than or equal to 1.4V, that is, Vout is infinitely close to 1.4V, it can be determined that the voltage applied to the same-direction end at this time is the target measurement value VIO. In extreme cases, if the initial measured value of Vout is no greater than 1.4V or is always greater than 1.4V, VIO can be set to 999.9999mV, and the chip is judged to be "damaged".

以下是在ASL-1000上自行开发的LM311的VIO测试程序Vio_2.cpp。
#include "asl.h"
#pragma warning (disable:4244)
#include "Vio_2.h"
void Vio_2_user_init(test_function&func)
{
Vio_2_params *ours;
ours=(Vio_2_params*)func.params;
}
void Vio_2(test_function&func)
{
Vio_2_params *ours;
ours=(Vio_2_params*)func.params;
float measured_V,Vadj,V,Vio;
int i;
board_hardware_init();
oal_8->open_relay(HV_BUF_CONN);
oal_8->open_relay(LOAD_REF_GND);
oal_8->close_relay(CONNECT_LOADS);
oal_8->close_relay(LOAD_REF_EXT);
oal_8->close_relay(LOAD_600);
dvi_9->set_current(DVI_CHANNEL_0,0.2);
dvi_11->set_current(DVI_CHANNEL_0,0.2);
dvi_9->set_voltage(DVI_CHANNEL_0,15); //VCC
dvi_11->set_voltage(DVI_CHANNEL_0, -15); //GND
delay(1);
oal_8->close_relay(DUT_OUT_OUT);
dvi_9->set_voltage_range(DVI_CHANNEL_1,POSITIVE_V_OUT,VOLT_20_RANGE, FAST_VOLTAGE_MODE); //set measure range
dvi_9->set_current(DVI_CHANNEL_1,1.0e-6);
dvi_9->set_meas_mode(DVI_CHANNEL_1,DVI_MEASURE_VOLTAGE);
func.dlog->power=POWER_MILLI;
Vadj=0.0;
V=0.0120;
for(i=0; i<40; i++)
{
Vadj=Vadj/3-3.0e-4;
dvi_11->set_voltage(DVI_CHANNEL_1,V);
dvi_11->set_current(DVI_CHANNEL_1,1.0e-3);
delay(1);
measured_V=dvi_9->measure();
if (measured_V>1.4000) V=V+Vadj;
else i=40;
}
if ((Vio==0.0120)||(measured_V>1.4000)) Vio=0.9999999;
else Vio=V;
do_dlog(func,0,Vio,ours->fail_bin,"");
board_hardware_init();
}

The open-loop test of the voltage comparator VIO mentioned above realizes the transformation from indirect test to direct test. Appropriately reducing the step (extending the test time) can improve the test resolution, but all the Force and Measure resources in the ASL-1000 configuration are used up. Therefore, when testing the dual comparator and quad comparator, all the input and output terminals of the device are connected to mux_20, and the serial test of each channel is completed through the internal relay matrix switching of the ASL-1000.