ltc6910-2 Linear Technology Corporation, ltc6910-2 Datasheet - Page 20

• Manufacturer Part Number: ltc6910-2
• Description: Digitally Controlled Programmable Gain Amplifiers In Sot-23
• Manufacturer: Linear Technology Corporation

LTC6910-1
LTC6910-2/LTC6910-3
the LTC6910-3 (gains 0V to 7V/V) it is 117dB. The SNR
from an amplifier is the ratio of input level to input-
referred noise, and can be 110dB with the LTC6910 family
at unity gain.
Construction and Instrumentation Cautions
Electrically clean construction is important in applications
seeking the full dynamic range of the LTC6910-X ampli-
fier. Short, direct wiring will minimize parasitic capaci-
tance and inductance. High quality supply bypass capaci-
tors of 0.1 F near the chip provide good decoupling from
a clean, low inductance power source. But several cm of
wire (i.e., a few microhenrys of inductance) from the
power supplies, unless decoupled by substantial capaci-
tance ( 10 F) near the chip, can cause a high-Q LC
resonance in the hundreds of kHz in the chip’s supplies or
ground reference. This may impair circuit performance at
those frequencies. A compact, carefully laid out printed
circuit board with a good ground plane makes a
significant difference in minimizing distortion. Finally,
equipment to measure amplifier performance can itself
introduce distortion or noise floors. Checking for these
limits with a wire replacing the chip is a prudent routine
procedure.
Expanding an ADC’s Dynamic Range
Figure 6 shows a compact data acquisition system for
wide ranging input levels. This figure combines an
LTC6910-X programmable amplifier (8-lead TSOT-23)
with an LTC1864 analog-to-digital converter (ADC) in an
TYPICAL APPLICATIO S
20
V
IN
AGND
U
1 F
3
Figure 6. Expanding an ADC’s Dynamic Range
LTC6910-X
2
5V
CONTROL
8
7
GAIN
0.1 F
4
6
5
1 499
270pF
8-lead MSOP. This ADC has 16-bit resolution and a maxi-
mum sampling rate of 250ksps. An LTC6910-1, for ex-
ample, expands the ADC’s input amplitude range by 40dB
while operating from the same single 5V supply. The 499
resistor and 270pF capacitor couple cleanly between the
LTC6910-X’s output and the switched-capacitor input of
the LTC1864. The 270pF capacitor should be an NPO or
X7R type, and lead length and inductance in the connec-
tions to the LTC1864 inputs must be minimized, to achieve
the full performance capability of this circuit. (See LTC
1864 data sheet for further general information.)
At a gain setting of 10V/V in an LTC6910-1 (digital input
100) and a 250ksps sampling rate in the LTC1864, a 10kHz
input signal at 60% of full scale shows a THD of
–87dB at the digital output of the ADC. 100kHz input
signals under the same conditions produce THD values
around – 75dB. Noise effects (both random and quantiza-
tion) in the ADC are divided by the gain of the amplifier
when referred to V
can acquire a signal that is 40dB down from full scale of
5V
an ADC alone (70 + 40 = 110dB of useful dynamic range at
250ksps), if available, would be far more expensive.
Low Noise AC Amplifier with Programmable Gain
and Bandwidth
Analog data acquisition can exploit band limiting as well as
gain to suppress unwanted signals or noise. Tailoring an
analog front end to both the level and bandwidth of each
source maximizes the resulting SNR.
P-P
with an SNR of over 70dB. Such performance from
V
IN
IN
GND
REF
LTC1864
+
–
CONV
SDO
SCK
V
6910 F04
CC
1 F
CONTROL
IN
ADC
in Figure 4. Because of this, the circuit
5V
6910123fa