The Quad Ramp and Hold Module is designed to measure input thresholds. The module enhances any input threshold test where an interface device requires relatively small input levels and has input hysteresis.

The module provides the capability to simultaneously locate input switching thresholds on up to four DUT inputs. Figure 10-1 shows a block diagram of the Quad Ramp and Hold Module.

Figure 10-2 is a pictorial overview of the function of the Quad Ramp and Hold Module within the Programmable Test Head (PTH), and its relation to the DUT and Test System (STS 3520, STS 4020 or STS 5020) hardware.

A hysteresis measurement is made by setting the input voltage to the negative supply voltage or ground. The input is then increased (ramped) toward the positive supply voltage as the output of the device is monitored for a change. When the device output swings, the input voltage is held constant and then measured. Then the input voltage is decreased toward the negative supply voltage (or ground) as the output of the device is monitored. When the output swings again, the input voltage is held constant and measured. The difference between the two measured values is the hysteresis and the average of the two measurements is the threshold voltage.

Refer to Figure 10-4 during the following discussion. A Quad Ramp and Hold Module contains four ramp and hold circuits. Each ramp and hold circuit consists of a programmable Ramp Generator controlled by a Voltage Comparator. The four Ramp Generators share one programmable Ramp Rate DAC (Digital-to-Analog Converter), one programmable Threshold Level DAC and common computer control over the slope, hold, and reset functions.

When the reset line is not asserted and the Hold Latch is clear, the capacitor in the Ramp Generator begins charging. When the DUT output transitions, the level is detected by the Comparator. The XOR Gate determines whether the transition slope matches the expected slope. If the slopes match, the Hold Latch is set. The output of the Hold Latch loops back to the Ramp Generator hold (switch) and the ramp voltage holds constant. There is a delay of 175 nanoseconds from when the input to the comparator crosses the programmed threshold to when the ramp actually stops. This delay produces an error term equal to the programmed dv/dt multiplied by 175 nanoseconds. The user should be aware of this error when using the QRH at higher slew rates.

The Digitizing Voltmeter (DVM) measures the ramp output level through the PTH Voltmeter Bus. The Ramp Generator Vcm (common mode voltage) input sets the ramp baseline voltage level. The Vcm input supplies the common mode voltage when testing devices such as comparators and bus receivers. These devices usually require an input threshold or offset at a common-mode voltage that is not zero. Vcm (Xn - 3 on the Module Interface Board) must be tied to a programmable voltage source (typically a Precision V/I Source or Bias Source) or ground.