AN-1219: Power-Off Protected Data Acquisition Signal Chain Using...

AN-1219
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Power-Off Protected Data Acquisition Signal Chain Using Fault-Protected CMOS
Switches
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
When signals come from remotely located sources, there is an
increased likelihood that damaging faults can occur. Overvoltage
conditions may occur due to systems with poorly designed
power supply sequencing or where hot-plug insertion is a
requirement. Transient voltages due to poor connections or
inductive coupling may damage components if not protected.
Faults can also occur due to power supply failures or in cases where
there is a loss of the power connection while switch inputs remain
connected to analog signals. Significant damage may result
from these fault conditions, which can mean expensive repairs.
Table 1. Devices Connected/Referenced
The circuit shown in Figure 1 provides protection to a data
acquisition signal chain using a low on-resistance ADG4612 quad
SPST with power off protection. The data acquisition system is
composed of the ADA4000-1 low cost, precision JFET input op
amp followed by an AD7476 low power, 12-bit, 1 MSPS SAR ADC.
The ADG4612 provides low cost protection against loss of power
while input signals are still present, as well as overvoltage fault
protection up to 16 V. The extra board area required for the
ADG4612 is minimal because the device is available in a 3 mm ×
3 mm LFCSP as well as a 16-lead TSSOP. The ADG4612 provides
protection for four separate data acquisition channels without
requiring any additional discrete components.
Product
ADG4612
ADA4000-1
AD7476
Description
Low on-resistance quad SPST switch with
power-off protection
Low cost, precision JFET input op amp
12-bit, 1 MSPS SAR ADC in a 6-lead SOT-23
Figure 1 shows a single channel, fault-protected data acquisition
signal chain that is made up of the ADG4612, ADA4000-1, and
AD7476. The key to protecting the data acquisition board is the
protection features of the ADG4612. The switch is in the isolation
mode when no power supplies are present, that is, when VDD is
floating or VDD ≤ 1 V or when the input signal, either VS or VD,
is greater than VDD supply plus a threshold voltage (VT). Under
these condition, the switch inputs are high impedance inputs,
ensuring that no current flows that can damage the switch or
downstream circuitry. The negative supply rail, VSS, can be floating
or 0 V to −5.5 V. The ground pin must be connected to the
ground potential.
The inputs of downstream components, such as the ADA4000-1
or AD7476, should be limited to within the supply rails so that,
when power supplies are lost or input signals exceed the supply
rails, these signals are blocked.
In the off condition, input signal levels of up to 16 V are blocked
(assuming VSS = 0 V). The switch also turns off when the analog
input signal level exceeds VDD by the threshold voltage, VT (~1.2 V).
OSCILLOSCOPE
SOURCE
DRAIN
+3.3V
+3.3V
+5V
RS
50Ω
6V p-p SINE WAVE
+3V DC OFFSET
FREQUENCY = 1kHz
IN1
S1
VDD
D1
VDD
V+
ADA4000-1
AD7476
ADG4612
4 × SPST
GND VSS
–3.3V
ADC
V–
VSS
–3.3V
INPUT RANGE =
0V TO +3.3V
09101-001
INPUT
Figure 1. Fault Protected Data Acquisition Signal Chain Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. A | Page 1 of 4
AN-1219
Application Note
The source, drain, and logic terminals of standard CMOS switches
have ESD protection in the form of clamping diodes to the supplies.
This is illustrated in Figure 2. These diodes vary in size depending
on the process but are generally designed to be small to minimize
leakage current. In normal operation, the diodes are reversed
biased and do not pass current. When forward biased, they are not
rated to pass currents greater than a few mA and can be damaged if
this current is exceeded. Whenever an analog switch input voltage
exceeds the supplies, the diodes become forward biased and
large currents can flow, meaning that the switch can be damaged,
even if the supplies are turned off. Furthermore, the damage
caused by a fault is not limited to the switch but can also affect
downstream circuitry such as the ADA4000-1 because applying
a signal to an unpowered ADA4000-1 exceeds the absolute
maximum rating of the device.
The waveform in Figure 3 shows the performance of a standard
analog switch when a signal is applied while the supplies are
floating. A 6 V p-p sine wave with a 3 V dc offset is applied to the
analog inputs, which in turn supply the power through the internal
ESD diodes powering up the switch and any other components
connected to the same VDD supply. The input signal passes
through the switch and is present at the ADA4000-1 inputs,
thereby exceeding the maximum ratings of the ADA4000-1.
6V
ANALOG
OUTPUT
(DRAIN)
0V
ANALOG INPUT
(SOURCE):
1kHz SINE WAVE
CH1 1.00V CH2 1.00V CH3 1.00V M200µs
A CH1
T
0.00000ms
VDD
VS > VD
FORWARD
CURRENT
FLOWS
Sx
Another limitation with standard CMOS switches is that, when
analog signals exceed the power supplies, VDD and VSS, the supplies
are pulled to within a diode drop of the fault signal. Internal
diodes become forward biased, and currents flow from the input
signal to the supplies. The fault signal can also pass through the
switch and damage parts downstream as shown in Figure 4.
Dx
RS
460mV
Figure 3. Standard Analog Switch with No Power Supply
LOAD
CURRENT
FORWARD
CURRENT
FLOATING VDD PULLED
HIGH BY SIGNAL INPUT
09101-003
For standard CMOS analog switches, power supply requirements
are specified in the product data sheet and should be adhered to
in order to guarantee optimal part performance and operation.
However, due to power supply failures, voltage transients, improper
sequencing, and system or user faults, it is not always possible to
meet data sheet recommendations.
RL
If the absolute maximum ratings of the devices are exceeded,
the long-term reliability may be affected.
GND
VSS
09101-031
VS
6V
Figure 2. ESD Protection on Standard Analog CMOS Switches
ANALOG INPUT
(SOURCE):
1kHz SINE WAVE
3.3V
ANALOG
OUTPUT
(DRAIN)
VDD
09101-004
0V
CH1 1.00V CH2 1.00V CH3 1.00V M200µs
A CH1
T
0.00000ms
460mV
Figure 4. Standard Analog Switch with Overvoltage Condition
Rev. A | Page 2 of 4
Application Note
AN-1219
The ADG4612 eliminates the effects of the faults previously shown.
There are no internal ESD diodes from the analog or digital inputs
to VDD or VSS. Instead, the ADG4612 uses other protection
components to protect against ESD events. This means that there
is no low impedance path to the supplies during loss of power
conditions or overvoltage faults. If signals are present on the
ADG4612 inputs before power is present, the switch enters
isolation mode, which means that the inputs have high impedance
paths to VDD, GND, and the output. This prevents current flow and
protects the device as well as downstream circuitry from damage.
6V
ANALOG INPUT
(SOURCE):
1kHz SINE WAVE
3.3V
0V
09101-005
VDD
460mV
Figure 5. ADG4612 with No Power Supply
Figure 6 shows the result of an overvoltage fault on the analog
inputs. In this case, a 6 V p-p sine wave with a 3 V dc offset is
applied to the ADG4612, which is powered from ±3.3 V supplies.
When the analog input exceeds VDD by a threshold voltage, VT
(~1.2 V), the switch enters isolation mode, thereby preventing
the fault from damaging downstream circuitry.
3.3V
ANALOG
OUTPUT
(DRAIN)
VDD
09101-006
0V
CH1 1.00V CH2 1.00V CH3 1.00V M200µs
A CH1
T
0.00000ms
Excellent layout, grounding, and decoupling techniques must be
used to achieve the desired performance from the circuits discussed
in this note (see MT-031 Tutorial and MT-101 Tutorial). As a
minimum, use a 4-layer PCB with one ground plane layer, one
power plane layer, and two signal layers.
COMMON VARIATIONS
Figure 7 shows a variation of the circuit shown in Figure 1 that
operates on a single 3.3 V supply. In this application, an op amp
with a rail-to-rail input and output is required to drive the AD7476
over its full input range. The outputs of the AD8655 op amp are
specified to drive within 10 mV to 30 mV of each supply rail. This
means that there are a small percentage of dead band codes at
each end of the full-scale ADC range where linearity suffers.
ANALOG INPUT
(SOURCE):
1kHz SINE WAVE
6V
Note that the input range of the AD7476 is equal to VDD, which
also acts as the reference to the ADC. In this case, the input range is
0 V to 3.3 V. To drive the AD7476 linearly over this range, the
supply voltages of the ADA4000-1 must be set slightly higher to
allow sufficient headroom for the output stage (approximately
1.2 V with respect to the positive supply and 2 V with respect to
the negative supply). This is achieved by setting the positive supply
voltage of the ADA4000-1 to +5 V, and the negative supply voltage
to −3.3 V. The two external Schottky diodes connected to the
AD7476 input ensure that there are no problems with power
supply sequencing.
For configurations other than an SPST, the ADG4613 is also
available; it has two switches with digital control logic similar
to that of the ADG4612; the logic is inverted on the other two
switches. Each switch conducts equally well in both directions
when on, and each switch has an input signal range that extends to
the supplies. The ADG4613 exhibits break-before-make switching
action for use in multiplexer applications. This device can be
configured as a quad SPST, a dual SPDT, or a 4:1 multiplexer to
suit different applications.
ANALOG
OUTPUT
(DRAIN)
CH1 1.00V CH2 1.00V CH3 1.00V M200µs
A CH1
T
0.00000ms
The combination of the ADG4612, ADA4000-1, and AD7476
shown in Figure 1 provides a data acquisition circuit that is robust
to fault conditions such as loss of power with signals from
external sources still present or overvoltage faults on analog inputs.
460mV
Figure 6. Overvoltage Condition on the ADG4612
For a 30 mV headroom requirement, this is approximately 1%
of the 3.3 V input range. Further discussions of op amp rail-torail issues, as well as overvoltage protection, can be found in the
MT-035 and MT-036 tutorials.
Also note that the ADG4612 operates with a VSS of 0 V in the
circuit and still maintains good on-resistance flatness over the
input signal range.
The AD8656 is a dual version of the AD8655. The ADA4000-2
and ADA4000-4 are dual and quad versions of the ADA4000-1.
Rev. A | Page 3 of 4
AN-1219
Application Note
OSCILLOSCOPE
SOURCE
DRAIN
+3.3V
+3.3V
+3.3V
RS
50Ω
IN1
S1
6V p-p SINE WAVE
+3V DC OFFSET
FREQUENCY = 1kHz
VDD
D1
VDD
V+
AD8655
AD7476
ADG4612
4 × SPST
GND VSS
ADC
V–
INPUT RANGE =
0V TO +3.3V
09101-007
INPUT
Figure 7. Single Supply Fault Protected Data Acquisition Signal Chain Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
LEARN MORE
Data Sheets and Evaluation Boards
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
ADG4612/ADG4613 Data Sheet
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues, Analog Devices.
ADA4000-1 Data Sheet
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection, Analog Devices.
ADA4000-4 Data Sheet
MT-088 Tutorial, Analog Switches and Multiplexers Basics,
Analog Devices.
AD7476 Evaluation Board
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
ADG4612 Evaluation Board (EVAL-ADG4612EBZ)
ADA4000-2 Data Sheet
AD7476 Data Sheet
REVISION HISTORY
4/13—Rev. 0 to Rev. A
Document Title Changed from CN-0165 to AN-1219 ....... Universal
1/11—Revision 0: Initial Version
©2011–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D09101-0-4/13(A)
Rev. A | Page 4 of 4
Similar pages