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Successive-Approximation
ADCs: Ensuring a Valid
First Conversion
By Steven Xie
Power Supply Sequencing
Introduction
Successive-approximation analog-to-digital converters (ADCs)
with up to 18-bit resolution and 10-MSPS sample rates meet the
demands of many data-acquisition applications, including portable,
industrial, medical, and communications. This article shows how to
initialize a successive-approximation ADC to get valid conversions.
Successive-Approximation Architecture
Successive-approximation ADCs comprise four main subcircuits:
the sample-and-hold amplifier (SHA), analog comparator, reference
digital-to-analog converter (DAC), and successive-approximation
register (SAR). Because the SAR controls the converter’s operation,
successive-approximation converters are often called SAR ADCs.
RESET
TIMING
ANALOG
INPUT
SW
• REF IN/REFOUT (AD765x-1)
• Analog Input Settling Time (AD7606)
• Analog Input Range (AD7960)
• Power-Down/Standby Mode (AD760x)
• Latency Delay (AD7682/AD7689, AD7766/AD7767)
• Digital Interfacing Timing
CONVERT
Data Access During Power Supply Ramp
EOC, DRDY,
OR BUSY
COMPARATOR
SHA
Some ADCs that operate with multiple supplies have well-defined
power-up sequences. The AN-932 Application Note, Power
Supply Sequencing, provides a good reference for designing
power supplies for these ADCs. Special attention should be
paid to the analog and reference inputs, as these typically
should not exceed the analog supply voltage by more than
0.3 V. Thus, AGND – 0.3 V < V IN < V DD + 0.3 V and
AGND – 0.3 V < VREF < V DD + 0.3 V. The analog supplies should
be turned on before the analog input or reference voltage, or the
analog core could power up in a latched-up state. In a similar
fashion, the digital inputs should be between DGND − 0.3 V
and V IO + 0.3 V. The I/O supply must be turned on before (or at
the same time as) the interface circuitry, or ESD diodes on these
pins could become forward-biased and power up the digital core
in an unknown state.
CONTROL LOGIC:
SUCCESSIVE
APPROXIMATION
REGISTER
(SAR)
DAC
Do not access the ADC before the power supplies are stable, as
this may put it into an unknown state. Figure 2 shows an example
where the host FPGA is trying to read data from an AD7367
while DVCC is ramping up, which may put the ADC into an
unknown state.
CNVST
OUTPUT
PARALLEL/SERIAL
Figure 1. Basic SAR ADC architecture.
After power-up and initialization, a signal on CONVERT starts the
conversion cycle. The switch closes, connecting the analog input to
the SHA, which acquires the input voltage. When the switch opens,
the comparator determines whether the analog input, which is now
stored on the hold capacitor, is greater than or less than the DAC
voltage. To start, the most significant bit (MSB) is on, setting the
DAC output voltage to midscale. After the comparator output has
settled, the successive-approximation register turns off the MSB
if the DAC output was larger than the analog input, or keeps it on
if the output was smaller. The process repeats with the next most
significant bit, turning it off if the comparator determines that
the DAC output is larger than the analog input, or keeping it on if
the output was smaller. This binary search continues until every
bit in the register is tested. The resulting DAC input is a digital
approximation of the sampled input voltage, and is output by the
ADC at the end of the conversion.
Factors Related To SAR Conversion Code
This article discusses the following factors as they relate to valid
first conversions:
• Power Supply Sequence (AD765x-1)
• Access Control (AD7367)
• RESET (AD765x-1/AD7606)
BUSY
CS
DVCC
Figure 2. Reading data during DVCC ramp-up.
SAR ADC Initialization with Reset
Many SAR ADCs, such as the AD760x and the AD765x-1,
require a RESET for initialization after power-up. After all power
supplies are stable, a specified RESET pulse should be applied
to guarantee that the ADC starts in the intended state, with
digital logic control in the default state and the conversion data
register cleared. Upon power up, voltage starts to build up on the
REF IN/REFOUT pin, the ADC is put into acquisition mode, and
the user-specified mode is configured. Once fully powered up, the
AD760x should see a rising edge RESET to configure it for normal
operation. The RESET high pulse should typically be 50 ns wide.
Establishing the Reference Voltage
The ADC converts the analog input voltage to a digital code
referred to the reference voltage, so the reference voltage
must be stable before the first conversion. Many SAR ADCs
have a REF IN/REFOUT pin and a REF or REFCAP pin. An
external reference can overdrive the internal reference via the
REF IN/REFOUT pin or the internal reference can drive the buffer
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Analog Dialogue 47-12, December (2013)1
directly. A capacitor on the REFCAP pin decouples the internal
buffer output, which is the reference voltage used for conversion.
Figure 3 shows a reference circuit example from the AD765x-1
data sheet.
REFCAPA
Analog Input Range
SAR
BUF
REFIN /
REFOUT
Make sure the analog input is within the specified input range,
taking special care of differential input ranges with a specified
common-mode voltage, as shown in Figure 5.
SAR
REF
SAR
BUF
REFCAPB
±5-V range to give the selected channel enough time to settle to
16-bit resolution. Front-End Amplifier and RC Filter Design
for a Precision SAR Analog-to-Digital Converter by Alan
Walsh (Analog Dialogue Volume 46, Number 4, 2012) provides
additional details regarding amplifier selection.
5V
SAR
IN+
0V
5V
SAR
BUF
SAR ADC
IN–
SAR
0V
REFCAPC
Figure 3. AD765x-1 reference circuit.
Make sure that the voltage on REF or REFCAP has settled before
the first conversion. The slew rate and settling time varies for
different reservoir capacitors, as shown in Figure 4.
Figure 5. Fully differential input with common-mode voltage.
For example, the AD7960 18-bit, 5-MSPS SAR ADC’s differential
input range is –V REF to +V REF, but both V IN+ and V IN– referred
to ground should be in the –0.1 V to V REF + 0.1 V range, and
the common-mode voltage should be around V REF/2, as shown
in Table 1.
Table 1. Analog Input Specifications for the AD7960
1
2
3
Test Conditions/
Comments
Min
Voltage Range
VIN+ − VIN−
VIN+, VIN−
to GND
Operating Input
Voltage
Common-Mode
Input Range
CAP A: 1F
CAP B: 10F
Max
Unit
−VREF
+VREF
V
−0.1
VREF + 0.1
V
VREF/
2 + 0.05
V
VREF/
2 − 0.05
Typ
VREF/2
Bringing the SAR ADC Out of Power-Down or Standby Mode
CAP C: 22F
CH1 500mV/DIV
1M BW: 20.0M
CH2 500mV/DIV
1M BW: 20.0M
CH3 500mV/DIV
1M BW: 20.0M
10.0ms/DIV 50.0kS/s
20.0s/pt
A CH3
1.25V
Figure 4. Voltage ramp on AD7656-1 REFCAPA/B/C
pins with different capacitors.
In addition, a poorly designed reference circuit can cause serious
conversion errors. The most common manifestation of a reference
problem is “stuck” codes, which may be caused by the size and
placement of the reservoir capacitor, insufficient drive strength, or
a large amount of noise on the input. Voltage Reference Design
for Precision Successive-Approximation ADCs by Alan Walsh
(Analog Dialogue Volume 47, Number 2, 2013) provides details
regarding reference design for SAR ADCs.
Analog Input Settling Time
For multichannel, multiplexed applications, the driver amplifier
and the ADC’s analog input circuitry must settle to the 16-bit
level (0.00076%) for a full-scale step on the internal capacitor array.
Unfortunately, amplifier data sheets typically specify settling to
a 0.1% or 0.01% level. The specified settling time could differ
significantly from the settling time at a 16-bit level, so verification
is required prior to driver selection.
Pay special attention to settling time in multiplexed applications.
After the multiplexer switches, make sure to allow enough time
for the analog input to settle to the specified accuracy before the
conversion starts. When using the AD7606 with a multiplexer,
allow at least 80 µs for the ±10-V input range and 88 µs for the
2
Parameter
To conserve power, some SAR ADCs go into power-down or
standby mode when they are idle. Make sure that the ADC comes
out of this low-power mode before the first conversion starts. For
example, the AD7606 family offers two power-saving modes: full
shutdown and standby. These modes are controlled by GPIO pins
STBY and RANGE.
Figure 6 shows that when STBY and RANGE return high, the
AD7606 goes from full shutdown mode into normal mode and
is configured for the ±10-V range. At this point, the REGCAPA,
R EGCAPB, and R EGCAP pins power up to the correct
voltages as outlined in the data sheet. When placed in standby
mode, the power-up time is approximately 100 μs, but it takes
approximately 13 ms in external reference mode. When powered
up from shutdown mode, a RESET signal must be applied after
the required power-up time has elapsed. The data sheet specifies
the time required between power-up and a rising edge on RESET
as tWAKE-UP SHUTDOWN.
VCC
VDRIVE
STBY
RANGE
REGCAP
tWAKE-UP SHUTDOWN tRESET
RESET
tDELAY
CONVST
Figure 6. AD7606 initialization timing.
Analog Dialogue 47-12, December (2013)
START OF CONVERSION
(SOC)
tCYC
POWER
UP
PHASE
tCONV
CONVERSION
(n – 2) UNDEFINED
EOC
EOC
EOC
EOC
tDATA
ACQUISITION
(n – 1) UNDEFINED
CONVERSION
(n – 1) UNDEFINED
CONVERSION
(n)
ACQUISITION
(n)
ACQUISITION
(n + 1)
CONVERSION
(n + 1)
ACQUISITION
(n + 2)
CNV
DIN
XXX
CFG (n)
CFG (n + 1)
CFG (n + 2)
RDC
DATA (n – 3)
XXX
SDO
SCK
1
17
DATA (n – 2)
XXX
1
DATA (n – 1)
XXX
1
17
DATA (n)
1
17
17
Figure 7. General timing for AD7682/AD7689.
SAR ADCs with Latency Delay
A common belief is that SAR ADCs have no latency delay, but
some SAR ADCs have a latency delay for configuration updates,
so the first valid conversion code may be undefined until the
latency delay—which may be several conversion periods—
has passed.
For example, the AD7985 features two conversion modes of
operation: turbo and normal. Turbo mode, which allows the
fastest conversion rate of up to 2.5 MSPS, does not power down
between conversions. The first conversion in turbo mode contains
meaningless data, and should be ignored. In normal mode, on
the other hand, the first conversion is meaningful.
For the AD7682/AD7689, the first three conversion results
after power-up are undefined, as a valid configuration does not
take place until after the second EOC. Therefore, two dummy
conversions are required, as shown in Figure 7.
When using the AD765x-1 in hardware mode, the logic state of the
RANGE pin is sampled on the falling edge of the BUSY signal to
determine the range for the next simultaneous conversion. After
a valid RESET pulse, the AD765x-1 defaults to operating in the
±4 × V REF range, with no latency problem. If, however, the
AD765x-1 operates in ±2 × V REF range, one dummy conversion
cycle must be used to select the range at the first falling edge
of BUSY.
In addition, some SAR ADCs, such as the AD7766/AD7767
oversampled SAR ADC, have postdigital filters that cause
additional latency delay. When multiplexing analog inputs to this
type of ADC, the host must wait the full digital filter settling time
before a valid conversion result can be achieved; the channel can
be switched after this settling time.
As shown in Table 2, the latency of the AD7766/AD7767 is 74
divided by the output data rate (74/ODR). When running at the
maximum output data rate of 128 kHz, the AD7766/AD7767
allows a 1.729-kHz multiplexer switching rate.
Table 2. Digital Filter Latency of AD7766/AD7767
Parameter
Test Conditions/
Comments
Group Delay
Settling Time (Latency)
Complete settling
Min
Typ
Max
Unit
37/ODR
µs
74/ODR
µs
Digital Interfacing Timing
Last, but not least, the host can access the conversion results
from SAR ADCs through some common interface options,
such as parallel, parallel BYTE, IIC, SPI, and SPI in daisychain mode. To get valid conversion data, make sure to follow
the digital interfacing timing specifications in the data sheet.
Conclusion
To get a first valid conversion code from SAR ADCs, please follow
the recommendations discussed in this article. Other specific
configuration support may be needed; consult the target SAR
ADC data sheet or application note for initialization before the
first conversion cycle starts.
References
Kester, Walt. Data Converter Support Circuits. Chapter 7, Data
Conversion Handbook.
Kester, Walt. “Which ADC Architecture Is Right for Your
Application?” Analog Dialogue, Volume 39, Number 2, 2005.
Walsh, Alan. “Front-End Amplifier and RC Filter Design for a
Precision SAR Analog-to-Digital Converter.” Analog Dialogue,
Volume 46, Number 4, 2012.
Author
Steven Xie [[email protected]] has worked as
an ADC applications engineer with the China Design
Center in ADI Beijing since March 2011. He provides
technical support for precision ADC products across
China. Prior to that, he worked as a hardware designer in the
Ericsson CDMA team for four years. In 2007, Steven graduated
from Beihang University with a master’s degree in communications
and information systems.
Analog Dialogue 47-12, December (2013)3
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