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Circuit Note
CN-0104
Circuit Designs Using Analog Devices Products
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Devices Connected/Referenced
AD5522
Quad Parametric Measurement Unit with
Integrated 16-Bit Level Setting DACs
AD7685 16-Bit, 250 kSPS PulSAR® ADC
ADR435
5 V Ultralow Noise XFET® Voltage
Reference
ADG1209
Low Capacitance, 4 Channel Differential
Multiplexer
ADG412 Quad SPST Switch
Parametric Measurement Unit and Supporting Components for ATE Applications
Using the AD5522 PMU and the AD7685 16-Bit ADC
Rev. 0
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CIRCUIT FUNCTION AND BENEFITS
This circuit is a quad parametric measurement unit (PMU) with
supporting components to service a minimum of four device-
under-test (DUT) channels. Typically, PMU channels are shared
among a number of DUT channels. Although the AD5522 is
very integrated and delivers four full PMU solutions, an
external reference and an ADC are required as a minimum to
complete this portion of the ATE signal chain. Typically, this
reference and the ADC can be shared among multiple PMU
packages. For further flexibility, additional external switches
can be used to extend the capabilities of the PMU by extending
the range of DUT capacitances that the AD5522 can drive.
CIRCUIT DESCRIPTION
The AD5522 quad PMU provides the forcing and measuring
functions for the DUT, but digitizing is required external to the
PMU. This can be achieved as follows:
• An ADC can be dedicated to each individual PMU
channel, providing the fastest throughput and result.
• An ADC can be shared across multiple channels. In
Figure 1, one AD7685 ADC is shared across the four PMU
channels. In some applications, an ADC might be shared
across many more channels, sometimes 8 or 16 PMU
channels.
The ADC can be shared across channels using the internal
Disable feature of each MEASOUTx pin. This requires a write
command to the PMU register to enable/disable the appropriate
switches. If this method is chosen, note that no more than one
MEASOUTx channel should be selected at any one time.
Alternatively, an external 4:1 multiplexer can be used to control
the measurement channel selection. In this way, all four
MEASOUTx paths can be enabled, and the multiplexer makes
the selection. Similarly, an 8:1 or 16:1 multiplexer would allow
more measurement paths to share the one ADC. The choice of
this multiplexer will depend on the ADC used and its input
voltage range. For bipolar input ADCs, one member of the
ADG1404/ADG1204 family would be ideal; for single-supply
usage, the ADG706 and ADG708 would be more suitable. The
output impedance of the MEASOUTx path is typically 60 Ω in
addition to the switch impedance. Therefore, an ADC buffer,
such as the ADA4898-1, should be considered to drive the ADC
(buffer not shown).
The AD7685 16-bit, 250 kSPS ADC was chosen for this
application because of its ability to handle the 0 V to 4.5 V
output range of the MEASOUTx path. In addition, the
availability of other ADCs with faster speeds in the same
footprint (for example, the 16-bit, 500 kSPS AD7686) also
makes it very attractive for upgrade paths.
The AD5522 requires a 5 V reference if a 20 V output range is
required. The ADR435 5 V XFET reference was chosen because
of its low tempco (10 ppm/°C, A Grade; 3 ppm/°C, B Grade),
low noise (8 µV p-p, 0.1 Hz to 10 Hz), and ability to drive
multiple PMU channels (30 mA source, 20 mA sink).
Some applications require the PMU to drive a wide range of
DUT capacitances, especially applications where the PMU is
connected to a power supply pin or where the PMU is used as a
device power supply and will see the decoupling/bypass
capacitance of the DUT. In such cases, an external switch
connected to the CCOMP pin, rather than a fixed capacitor,
allows for additional CCOMP capacitors to be switched in and
out, thereby allowing for optimization of settling time and
stability with various capacitive loads. The switch chosen for
this circuit was the ADG412 quad SPST, which has an on-
resistance of less than 50 Ω. The quad SPST switch was chosen
