DN1034 - Low Power, Precision Op Amp Simplifies Driving of MUXed ADCs

Low Power, Precision Op Amp Simplifies
Driving of MUXed ADCs
Design Note 1034
Guy Hoover
Introduction
The high speed op amps required to buffer a modern
16‑/18-bit analog-to-digital converter (ADC) typically
dissipate as much power as the ADC itself, often with
a maximum offset spec of about 1mV, well beyond
that of the ADC. If multiple multichannel ADCs are
required, the power dissipation can quickly rise to
unacceptable levels.
The simple buffer presented here is capable of driving
the LTC ®2372-18 8-channel ADC and achieving near
data sheet SNR, THD and offset performance with very
low power dissipation if the input signals involved are
in the range of DC to 1kHz.
Circuit Description
The LTC2372-18 is a low noise, 500ksps, 8-channel
18‑bit successive approximation register (SAR)ADC.
Operating from a single 5V supply, the LTC2372‑18
achieves –110dB THD (typical), 100dB (fully
differential)/95dB (pseudo-differential) SNR (typical)
with an offset of ±11LSB (maximum) while dissipating
only 27mW (typical).
The LT®6016 is a dual rail-to-rail input op amp with
input offset voltage less than 50µV (maximum) that
draws only 315µA per amplifier (typical). It is also
available as a single and a quad (LT6015/LT6017).
The circuit of Figure 1 shows the LT6016 op amp
configured as a noninverting buffer driving the analog
inputs of the LTC2372-18. Typical power dissipation of
each op amp is only 3.7mW. For all eight channels this
is a power dissipation of only 30mW, approximately
the same power dissipation as the ADC. Running the
LT6016 on a single 5.25V supply and enabling the
ADC’s digital gain compression mode reduces the
total op amp power consumption by more than half, to
13mW, at the expense of a slight decrease in the SNR.
The RC filter at the buffer output minimizes the noise
contribution of the LT6016 and reduces the effect of
the sampling transient caused by the MUX and the
input sampling capacitor.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
8V
0.1µF
4.096V
5V
1.8V TO 5V
0V
+
4300pF
4.096V
+
91Ω
0V
4.096V
0V
2.048V
MUXOUT–
LT6016
–
CH0
CH1
CH2
CH3 MUX
CH4
CH5
CH6
CH7
COM
4300pF
–3.6V
0.1µF
VDD
0.1µF
VDDLBYP OVDD
LTC2372-18
+
18-BIT
SAMPLING ADC
–
ADCIN–
0V
91Ω
2.2µF
ADCIN+
4.096V
–
MUXOUT+
0V
10µF
REFBUF
47µF
REFIN
GND
0.1µF
COM = GND
OR REFBUF/2
Figure 1. LT6016 Buffer Driving the LTC2372-18 8-Channel SAR ADC
10/15/DN1034f
RESET
RDL
SDO
SCK
SDI
BUSY
CNV
DN1034 F01
SAMPLE
CLOCK
Circuit Performance
Figure 2 shows a 32768-point FFT of the LTC2372‑18
driven fully differentially by the circuit of Figure 1. THD
is –114dB and SNR is 98.5dBFS at 400ksps, which
compares well with the typical specs of the LTC2372-18.
0
AIN = –1dBFS
fS = 400ksps
fIN = 1kHz
THD = –114dB
SNR = 98.5dB
–20
AMPLITUDE (dBFS)
–40
–60
Figure 4 shows THD vs sampling rate with digital gain
compression off and on for both pseudo-differential
and fully differential modes of the LTC2372-18. Here
THD starts to rise above –110dB at 300ksps for pseudodifferential mode and rises above –115dB at 400ksps
for fully differential mode. Digital gain compression has
only a minimal effect on the THD performance. In fully
differential mode, THD is never worse than –100dB up
to the full 500ksps sampling rate of the LTC2372-18.
–80
–80
–85
–100
–90
–120
–95
THD (dB)
–140
–160
–180
0
–100
–105
–115
DN1034 F02
–120
Figure 2. 32768-Point FFT for the Circuit of Figure 1
–125
Figure 5 shows the combined offset error of the buffer
and ADC vs sampling rate in pseudo-differential mode
with digital gain compression off. Offset is initially less
than 3LSB and does not degrade until the sampling
rate reaches 400ksps.
20
18
96
16
94
14
92
OFFSET (LSB)
SNR (dBFS)
50 100 150 200 250 300 350 400 450 500
SAMPLING RATE (ksps)
DN1034 F04
98
90
88
86
fIN = 1kHz
AIN = –1dBFS
R = 91Ω
C = 4300pF
84
82
0
100
PD
FD
FD GC
PD GC
200
300
400
SAMPLING RATE (ksps)
500
DN1034 F03
Figure 3. SNR vs Sampling Rate for the Circuit
of Figure 1 in Pseudo-Differential and Fully
Differential Modes
10/15/DN1034f
0
Figure 4. Pseudo-Differential, Fully Differential
THD vs Sampling Rate for the Circuit of
Figure 1 with and without Gain Compression
100
80
PD
FD
FD GC
PD GC
–110
20 40 60 80 100 120 140 160 180 200
FIN (kHz)
Figure 3 shows SNR vs sampling rate with digital gain
compression off and on for both pseudo-differential
and fully differential modes of the LTC2372-18. With
digital gain compression off, the supply voltage for the
LT6016 is +8V/–3.6V. With digital gain compression on,
the LT6016 runs off a single 5V supply. SNR stays fairly
flat at 94dBFS (pseudo-diff)/98.5dBFS (fully diff) with
digital gain compression off, and 92.1dBFS(pseudodiff)/96.6dBFS(fully diff) with digital gain compression
on, up to 500ksps for all modes.
fIN = 1kHz
AIN = –1dBFS
R = 91Ω
C = 4300pF
12
10
8
6
4
2
0
0
100
200
300
400
SAMPLING RATE (ksps)
500
DN1034 F05
Figure 5. Offset Error vs Sampling Rate for the
Circuit of Figure 1 in Pseudo-Differential Mode
Figure 6 shows distortion vs input frequency for a
400ksps sampling rate. Above 1kHz, distortion rises
for all modes.
–80
–85
THD (dB)
–90
fSMPL = 400ksps
AIN = –1dBFS
R = 91Ω
C = 4300pF
PD
FD
FD GC
PD GC
–95
–100
–105
–110
–115
–120
100
1k
INPUT FREQUENCY (Hz)
10k
DN1034 F06
Figure 6. Distortion vs Input Frequency for the
Circuit of Figure 1
Data Sheet Download
www.linear.com/LTC2372-18
Linear Technology Corporation
Conclusion
A simple driver for the LTC2372-18 18-bit, 500ksps,
8-channel SAR ADC—consisting of the LT6016 low
power precision dual op amp configured as noninvert‑
ing buffer is demonstrated. The driver dissipates only
3.7mW per op amp (typical), and can be reduced to
1.6mW by running off a single 5V supply with the ADC
in digital gain compression mode.
At sampling rates less than 300ksps, SNR is mea‑
sured at 94dB (pseudo-diff)/98.5dB (fully diff)
with gain compression off and 92.1dBFS (pseudodiff)/96.6dBFS(fully diff) with digital gain compression
on; THD is measured at –110dB (pseudo-diff)/–115dB
(fully diff) with digital gain compression off or on.
Offset measures less than 3LSB (pseudo-diff) with
gain compression off. Above 300ksps, performance
gradually declines up to the full 500ksps sampling
rate of the LTC2372-18.
For applications help,
call (408) 432-1900, Ext. 3227
DN1034f LT 1015 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2015
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