518 and probes 1250 and electrical connections 1244 electrically connect compare channels 544 and 546 with probes 1252 in this example probe 1254 is a calibration probe used for calibration and it is disposed to contact calibration output terminal 1224 on calibration substrate as mentioned above a calibration channel may be a channel normally used to provide power or ground during testing a channel that corresponds to an unused probe a compare channel a channel that corresponds to an input and or output terminal on a dut that does not require calibration or that is calibrated in a separate step or procedure or any channel that is used to return data to the tester 401 for calibration of another channel similarly a u201c calibration probe u201d may be a probe that corresponds to any such calibration channel as shown in fig1 calibration probe 1254 connects to calibration channel 548 through a buffer 1232 filter 1280 and detector 1236 detector 1236 may include an analog to digital detector not shown so that its output is in digital format calibration substrate 1212 includes a summing junction 1230 which may be similar to summing junction 580 in fig5 and 11 that combines signals from input terminals 1220 and outputs the combined signal to output terminals 1222 and calibration output terminal 1224 a set of probes 1250 1252 and 1254 electrical connections 1214 and 1244 and circuit elements 1232 1280 and 1236 may be included for each channel group e g 420 422 and 424 in tester 401 see fig4 resistors 1228 and zigs zags or curves 1226 may be generally similar to resistors 528 and zigs zags or.