DN526 - Driver for 14-Bit, 4.5Msps ADC Operates Over a Wide Gain Range

Driver for 14-Bit, 4.5Msps ADC Operates Over a Wide
Gain Range
Design Note 526
Guy Hoover
Introduction
The LTC ®2314-14 is a 14-bit, 4.5Msps, serial output
ADC with an integrated high performance reference.
The single-ended input of the LTC2314-14 is easy
to drive and in many instances does not require a
buffer. A driver, such as the LT6236 op amp, may be
required for a signal that is small or has high output
impedance. The LT6236 is a 215MHz gain bandwidth
product, rail-to-rail output op amp/SAR ADC driver that
features 1.1nV/√Hz input-referred noise voltage density
and draws only 3.5mA of supply current with a typical
offset of only 100µV. The LT6236 is a good choice for
these applications because of its high bandwidth, low
noise, low supply current and low offset.
ten depending on the value of R3, as shown in Table 1.
Capacitor C1 and resistor R1 limit the input bandwidth
to 68MHz. C1 also acts as a charge reservoir for the
ADC sample-and-hold capacitor and helps to isolate the
LT6236 from the transient that occurs at AIN when the
ADC goes into sample mode. R1 is also used to help
isolate the op amp from the ADC sampling transient.
Table 1. Gain and Input Range Settings for Various
Values of R3
The driver presented here is characterized over a range
of gains, sampling frequencies and input frequencies
to establish its application suitability.
R3
(Ω)
GAIN
AIN RANGE
(V)
∞
1
0 to 4.096
2k
2
0 to 2.048
499
5
0 to 0.8192
221
10
0 to 0.4096
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.
Driver Operation
Figure 1 shows a non-inverting amplifier driving the
LTC2314-14. The driver has a gain between one and
C6
2.2µF
+
C3
0.1µF
R1
49.9Ω
V+
U2
LT6236 OUT
–
– V
–3.6V
3.3V
C5
2.2µF
8V
AIN
0V TO 4.096V
MAX
5V
C2
0.1µF
R2
2k
C4
22pF
C7
2.2µF
1
2
5
VDD REF OVDD
4
C1
47pF
U1
LTC2314-14
AIN
GND
CS
SCK
SDO
3
dn4gh F01
R3
*
*SEE TABLE 1
Figure 1. Single-Ended ADC Driver with Up to 0V to 4.096V Input Range
05/14/526
8
CS
7
SCK
6
SDO
Driver Performance
The FFT in Figure 2 shows that with a gain of one
this circuit has an SNR of 77dB and a THD of –84dB,
with a sampling rate of 4.5Msps and a 600kHz input
frequency. These numbers are close to the typical data
sheet performance specifications for the LTC2314-14
alone, indicating that there is minimal performance
degradation of the ADC when using this driver. Figure 3 shows SINAD performance vs sampling rate for
gains of one through ten. The SINAD remains about
the same for all gains at approximately 75dB to 76dB,
with sampling rates from 1Msps to 4.5Msps, which is
the maximum rated sampling frequency for this part.
0
fS = 4.5Msps
fIN = 600kHz
SNR = 77dB
THD = 84dB
AV = 1
–40
80
70
66
60
–60
–80
55
–100
50
–120
–140
0
0.6
1.2
1.8
FREQUENCY (MHz)
2.4
Figure 2. 16k-Point FFT for the Circuit of Figure 1
80
75
70
SINAD (dB)
AV = 1
AV = 2
AV = 5
AV = 10
0
500
1500
1000
2000
INPUT FREQUENCY (kHz)
2500
dn4gh F04
dn4gh F02
66
60
AV = 1
AV = 2
AV = 5
AV = 10
55
50
fS = 4.5Msps
AIN = –1dBFS
75
SINAD (dB)
AMPLITUDE (dBFS)
–20
Figure 4 shows SINAD performance vs input frequency
for gains of one through ten. The SINAD starts at 75dB
to 76dB for an input frequency of 100kHz but falls as
the input frequency and gain rises. For most applications a drop in SINAD of 3dB should be acceptable.
At a gain of one, SINAD is reduced to 73dB beyond
2.2MHz. At a gain of two, SINAD is reduced to 73dB
at approximately 1.2MHz. At a gain of five, SINAD is
reduced to 73dB at approximately 600kHz. At a gain of
ten, SINAD is reduced to 73dB at approximately 250kHz.
1
0
Figure 4. SINAD vs Input Frequency
Conclusion
The LTC2314-14, 4.5Msps, 14-bit serial sampling ADC
can be driven by the LT6236 rail-to-rail output, 215MHz
low noise op amp/SAR ADC driver with gains ranging
from one through ten, sample rates from 1Msps to
4.5Msps and input frequencies ranging from 100kHz
to 2.2MHz. This driver circuit is appropriate for small
or high output impedance signals. The results shown
in Figures 3 and 4 can be used to assess the suitability
of this driver for an application with a particular input
signal bandwidth and gain.
fIN = 100kHz
AIN = –1dBFS
3
2
4
SAMPLING RATE (Msps)
5
dn4gh F03
Figure 3. SINAD vs Sampling Rate
Data Sheet Download
www.linear.com/LTC2314-14
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn526f LT/AP 0514 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2014
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