ADA4940-1 AD [Analog Devices], ADA4940-1 Datasheet - Page 26

• Manufacturer Part Number: ADA4940-1
• Description: Ultralow Power, Low Distortion
• Manufacturer: AD [Analog Devices]

ADA4940-1/ADA4940-2
DRIVING A HIGH PRECISION ADC
The
dc-coupled applications. The circuit in Figure 73 shows a front-
end connection for an
an 18-bit, 1 MSPS successive approximation, analog-to-digital
converter (ADC) that operates from a single power supply, 3 V
to 5 V. It contains a low power, high speed, 18-bit sampling
ADC and a versatile serial interface port. The reference voltage,
REF, is applied externally and can be set independent of the
supply voltage. As shown in Figure 73, the
coupled on the input and the output, which eliminates the need
for a transformer to drive the ADC. The amplifier performs a
single-ended-to-differential conversion if needed and level
shifts the input signal to match the input common mode of the
ADC. The
(+6 V and −1 V) and a gain that is set by the ratio of the
feedback resistor to the gain resistor. In addition, the circuit
can be used in a single-ended-input-to-differential output or
differential-input-to-differential output configuration. If needed,
a termination resistor in parallel with the source input can be
used. Whether the input is a single-ended input or differential,
the input impedance of the amplifier can be calculated as shown in
the Terminating a Single-Ended Input section. If R1 = R2 = R3 =
R4 = 1 kΩ, the single-ended input impedance is approximately
1.33 kΩ, which, in parallel with a 52.3 Ω termination resistor,
provides a 50 Ω termination for the source. An additional 25.5 Ω
(1025.5 Ω total) at the inverting input balances the parallel
impedance of the 50 Ω source and the termination resistor driving
the noninverting input. However, if a differential source input is
used, the differential input impedance is 2 kΩ. In this case, two
52.3 Ω termination resistors are used to terminate the inputs.
In this example, the signal generator has a 10 V p-p symmetric,
ground-referenced bipolar output. The V
noise reduction and set externally with 1% resistors to 2.5 V to
maximize the output dynamic range. With an output common-
ADA4940-1/ADA4940-2
ADA4940-1
ADA4940-1
is configured with a dual 7 V supply
R5
R6
+D
–D
are ideally suited for broadband
IN
IN
0.1µF
R3
R1
driving an AD7982, which is
OCM
ADA4940-1
+IN
V
–IN
input is bypassed for
OCM
R2
R4
Figure 73.
ADA4940-1
ADA4940-1
is dc-
–FB
+FB
Rev. B | Page 26 of 32
–1V
+6V
(LFCSP) Driving the
–OUT
+OUT
mode voltage of 2.5 V, each
V and 5 V, opposite in phase, providing a gain of 1 and a 10 V p-
p differential signal to the ADC input. The differential RC section
between the
low-pass filtering with a corner frequency of 1.79 MHz and extra
buffering for the current spikes that are output from the ADC input
when its sample-and-hold (SHA) capacitors are discharged.
The total system power in Figure 73 is under 35 mW. A large
portion of that power is the current coming from supplies to the
output, which is set at 2.5 V, going back to the input through the
feedback and gain resistors. To reduce that power to 25 mW,
increase the value of the feedback and gain resistor from 1 kΩ
to 2 kΩ and set the value of the resistors R5 and R6 to 3 kΩ. The
ADR435
up powering the ADC and setting the reference voltage for the
V
Figure 72 shows the fft of a 20 kHz differential input tone
sampled at 1 MSPS. The second and third harmonics are down
at −118 dBc and −122 dBc.
OCM
Figure 72. Distortion Measurement of a 20 kHz Input Tone (CN-0237)
33Ω
33Ω
–100
–120
–140
–160
pin.
–20
–40
–60
–80
AD7982
0
0
is used to regulate the +6 V supply to +5 V, which ends
2.7nF
2.7nF
ADA4940-1
10µF
ADC
20k
IN+
IN–
ADR435
GND
+6V
REF
AD7982
output and the ADC provides single-pole,
+5V
ADA4940-1
FREQUENCY (Hz)
40k
INTERFACE
+2.5V
SERIAL
VDD
60k
output swings between 0
80k
Data Sheet
100k