Texas Instruments | Circuit to drive a switched capacitor reference input for high-precision devices | Application notes | Texas Instruments Circuit to drive a switched capacitor reference input for high-precision devices Application notes

Texas Instruments Circuit to drive a switched capacitor reference input for high-precision devices Application notes
Analog Engineer's Circuit: Data
Converters
SBAA390 – June 2019
Circuit to drive a switched capacitor reference input for
high-precision devices
Art Kay
Power Supplies
REF6050 Input Voltage
Vref (Output of REF,
Input on ADC)
AVDD (ADS8860)
DVDD (ADS8860)
5.5V
5.0V
3.0V
3.0V
Design Description
This circuit document shows how the REF6050 (REF60xx series) voltage reference can be used to
directly drive a switched capacitance ADC reference input pin. The reference input pin on many different
types of data converters has a switched capacitor circuit that generates significant transient current
requirements. These transients can have a significant impact in ADC accuracy, because the voltage
reference must be settled to a constant accurate voltage. To respond to the transients on the ADC
reference input, the voltage reference needs a wide-bandwidth buffer. The REF60xx series voltage
reference incorporates an integrated wide-bandwidth buffer. This reference design will show measured
ADC performance comparing a low-bandwidth reference to the wide-bandwidth REF6050. The document
also covers alternative solutions for driving the ADC reference input.
R1 1k
5.5V
VIN
VIN
EN
Bandgap
Voltage
Reference
RFLT
OUT_S
OUT_F
+
FLT
RESR
100mŸ
REF6050
CFLT
1uF
CL
22uF
REFP
5.5V
+
-
+
Vin
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ADS8860
1-MSPS
GND
Circuit to drive a switched capacitor reference input for high-precision
devices
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Specifications
Specification (at 1MSPS)
Goal
Measured
THD
–108dB
–112.6dB
Reference Settling
0 codes
1.2 codes
Design Notes
1. This circuit can apply to many different data converters. See the data sheet to determine if a widebandwidth buffer is required. For devices that have a switched capacitor input, the REF60xx series
reference is a good solution because it has an integrated wide-bandwidth buffer. For data converters
with an integrated wide-bandwidth buffer, the reference bandwidth is not important; therefore, consider
other parameters such as reference noise and drift.
2. See the REF60xx (REF6050) High-Precision Voltage Reference With Integrated ADC Drive Buffer
Data Sheet for selecting the filter capacitors. In general, choosing larger capacitors helps to minimize
noise. Also, the reference stability can be impacted by the Equivalent Series Resistance (ESR) of the
capacitor.
3. The topic of reference drive is covered in detail in the Driving the Reference Input on a SAR ADC
video series from the TI Precision Labs - ADCs video training. The measurements shown in this
reference design can be reproduced using the PLABS-SAR-EVM-PDK.
2
Circuit to drive a switched capacitor reference input for high-precision
devices
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Wide-Bandwidth Buffer Inside the Reference
Many different data converters have a switched capacitor reference input. The following image shows a
very simple model of this input. This model is for a traditional SAR ADC, but a similar model exists for
some delta-sigma converters. In this example, a capacitance Cx is connected and disconnected from the
external reference via SWBIT. The capacitor is reset with SWx after it is disconnected. This process is
repeated N-times for an N-bit converter. This creates a large transient input current each time the
capacitor is connected to the reference. The transient can be milliamps in amplitude, and the time
between the transients is nanoseconds. The value of Cx changes throughout the conversion cycle, so the
amplitude of the transients changes (see the following reference input graph). To get accurate conversion
results, the reference voltage must rapidly respond to these transients. Thus, the reference requires a
wide-bandwidth buffer to achieve the highest accuracy. Most references have an internal low bandwidth
buffer which may not be adequate for the best performance. The REF6050 incorporates an internal widebandwidth buffer which is capable of responding to the transients present on most ADCs with a switchedcapacitor reference input.
R1 1k
5.5V
VIN
VIN
EN
Bandgap
Voltage
Reference
RFLT
OUT_S
OUT_F
+
FLT
RESR
100mŸ
REF6050
CFLT
1uF
REFP
SWBIT
CL
22uF
SWX
CX
Reference
Input
Model
GND
ADS8860
1-MSPS
Reference Input
Current Transients (mA)
Convert Start Signal
Bit Transients
16 Transients for a 16-bit Converter
Pulses are Nanoseconds
From Each Other
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Wide-Bandwidth Buffer Inside the ADC
Some data converters have an internal wide-bandwidth buffer that is designed to drive the switched
capacitor reference input. The input of this buffer is high impedance so the reference does not require a
wide-bandwidth output. The ADS8900B and ADS8168 devices are examples of data converters that
incorporate an integrated wide-bandwidth reference buffer.
+5.3V
0.1Ÿ
1kŸ
Vin
22µF
+5V
Vout
1uF
10uF
220mŸ
REFIN
REFBUFOUT
GND
22uF
BUF
REF5050
5V Reference
ADS8900B
1-MSPS
ADC
Discrete Wide-Bandwidth Buffer Between Reference and ADC
For devices that require a buffer, it is possible to build a discrete reference buffer rather than using a
device with in integrated buffer (for example, REF6050). The following circuit shows a discrete buffer used
on the ADS9110EVM-PDK. This circuit is a composite amplifier because it combines two amplifiers to
form a circuit that has the benefit of both devices. The OPA378 has an ultra-low offset, and the OPA625
can respond to rapid transients.
Vref
4.5V
30k
10p
1k
499
100n
-
2.49k
U2
OPA625
10uF
220n
M
+
0.1uF
4.99k
100n
+
U3
OPA378
5V
+
4.7
5V
+5.3V
Vout
REFP
ADS9110
2-MSPS
Vin
1uF
GND
GND
REF5045
4.5V Reference
4
Circuit to drive a switched capacitor reference input for high-precision
devices
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Distortion With and Without Reference Buffer Versus Sampling Rate
The following graph compares the distortion for an ADC with and without a wide-bandwidth reference
buffer. Notice that for high sampling rates, the ADC with the wide-bandwidth buffer has substantially better
THD performance. At low sampling rates, the buffer is less important because the reference has
significant time to recover from the reference transients.
Distortion vs Sample Frequency for Buffered
Distortion
vs Sample Freq.
and Unbuffered References
-90
-90
Distortion (dB)
-95
REF5050
No WideBandwidth Buffer
-100
-100
-105
-110
-110
REF6050
Integrated WideBandwidth Buffer
-115
-120
-120
10
10k
100
100k
Sampling Frequency (Hz)
1000
1M
FFT With and Without Reference Buffer at 1MSPS
The following graphs show the FFT with and without the wide-bandwidth reference buffer at the maximum
sampling rate (1MSPS). The graphs show that the harmonic distortion is clearly larger for the ADC without
the wide-bandwidth buffer.
ADS8860 + REF6050 at Maximum
Sampling Rate
Low Distortion, THD í112.6dB
ADS8860 + REF5050 at Maximum
Sampling Rate
High Distortion, THD í93.5dB
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Reference Settling With and Without Reference Buffer Versus Sampling Rate
The following graph compares the settling performance to a DC signal for an ADC with and without a
wide-bandwidth reference buffer. Settling performance indicates the error in the digitized result after one
ADC conversion. In subsequent conversions, the ADC settling error will reduce for a constant DC input.
Settling performance is very important in multiplexed systems, because it provides a good estimate of the
types of error when switching channels. Notice that for high sampling rates, the ADC with the widebandwidth buffer has substantially better settling performance. At low sampling rates, the buffer is less
important because the reference has significant time to recover from the reference transients.
Reference Settling vs Sample Frequency
for Buffered and Unbuffered References
Reference Settling in Bits
12
12
10
10
8
REF5050
No WideBandwidth Buffer
6
REF6050
Integrated WideBandwidth Buffer
4
2
0
10k
10
100k
100
Sampling Frequency (Hz)
1M
1000
Reference Settling With and Without Reference Buffer Versus Time
The following graph shows the reference settling versus time at maximum sampling rate with a DC signal
applied. Notice that for the unbuffered reference the signal has a significant transient settling error of 9.55
codes. This error requires about 300 samples to fully settle.
ADS8860 + REF6050 at Maximum Sampling Rate
Low-Reference Droop, THD = 1.2 codes
ADS8860 + REF5050 at Maximum Sampling Rate
High-Reference Droop, THD = 9.55 codes
6
Circuit to drive a switched capacitor reference input for high-precision
devices
Copyright © 2019, Texas Instruments Incorporated
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Design Featured Devices
Device
Key Features
REF6050
integrated ADC drive buffer, low output impedance: < 50 mΩ
http://www.ti.com/product/REF6050
(0–200kHz), excellent temperature drift performance 5 ppm/°C (max) ,
extremely low noise total noise: 5µVRMS with 47-µF capacitor
Link
Other Possible Devices
http://www.ti.com/vref
ADS8860
16-bit resolution, SPI, 1MSPS sample rate, single-ended input, VREF
input range 2.5 V to 5.0 V
http://www.ti.com/product/ADS8860
http://www.ti.com/adcs
OPA320
20-MHz bandwidth, rail-to-rail with zero crossover distortion,
VOS(MAX) = 150μV, VOS(Drift MAX) = 5μV/°C, en = 7nV/√Hz
http://www.ti.com/product/OPA320
http://www.ti.com/opamps
References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
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Circuit to drive a switched capacitor reference input for high-precision
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