For applications that use op amps to drive the ADC, it is
recommended that a series resistor be added between the
amplifier output and the converter inputs. This will isolate the
capacitive input of the converter from the driving source and
avoid gain peaking, or instability; furthermore, it will create a
1st-order, low-pass filter in conjunction with the specified
input capacitance of the ADS5421. Its cutoff frequency can
be adjusted further by adding an external shunt capacitor
from each signal input to ground. The optimum values of this
RC network, however, depend on a variety of factors, includ-
ing the ADS5421 sampling rate, the selected op amp, the
interface configuration, and the particular application (time
domain versus frequency domain). Generally, increasing the
size of the series resistor and/or capacitor will improve the
SNR, however, depending on the signal source, large resis-
tor values may be detrimental to the harmonic distortion
performance. In any case, the use of the RC network is
optional but optimizing the values to adapt to the specific
application is encouraged.
datasheet located at the Texas Instruments web site
(www.ti.com). In general, differential amplifiers provide for a
high-performance driver solution for baseband applications,
and different differential amplifier models can be selected
depending on the system requirements.
TRANSFORMER-COUPLED INTERFACE CIRCUITS
If the application allows for AC-coupling but requires a signal
conversion from a single-ended source to drive the ADS5421
differentially, using a transformer offers a number of advan-
tages. As a passive component, it does not add to the total
noise, and by using a step-up transformer, further signal
amplification can be realized. As a result, the signal swing of
the amplifier driving the transformer can be reduced, leading
to an increased headroom for the amplifier and improved
distortion performance.
A transformer interface solution is given in Figure 4. The
input signal is assumed to be an IF and bandpass filtered
prior to the IF amplifier. Dedicated IF amplifiers are com-
monly fixed-gain blocks and feature a very high bandwidth,
low-noise figure, and a high intercept point, but at the
expense of high quiescent currents, which are often around
100mA. The IF amplifier may be AC-coupled, or directly
connected to the primary side of the transformer. A variety of
miniature RF transformers are readily available from different
manufacturers, (e.g., Mini-Circuits, Coilcraft, or Trak). For
selection, it is important to carefully examine the application
requirements and determine the correct model, the desired
impedance ratio, and frequency characteristics. Furthermore,
the appropriate model must support the targeted distortion
level and should not exhibit any core saturation at full-scale
voltage levels. The transformer center tap can be directly tied
to the CM pin of the converter because it does not appreciably
load the ADC reference (see Figure 4). The value of termina-
tion resistor RT must be chosen to satisfy the termination
requirements of the source impedance (RS). It can be calcu-
lated using the equation RT = n2 • RS to ensure proper
impedance matching.
ANALOG INPUT DRIVER CONFIGURATIONS
The following section provides some principal circuit sugges-
tions on how to interface the analog input signal to the
ADS5421. Applications that have a requirement for DC-
coupling a new differential amplifier, such as the THS4502,
can be used to drive the ADS5421, as shown in Figure 3. The
THS4502 amplifier allows a single-ended to differential con-
version to be performed easily, which reduces component
cost. In addition, the VCM pin on the THS4502 can be directly
tied to the common-mode pin (CM) of the ADS5421 in order
to set up the necessary bias voltage for the converter inputs.
As shown in Figure 3, the THS4502 is configured for unity
gain. If required, higher gain can easily be configured, and a
low-pass filter can be created by adding small capacitors
(e.g., 10pF) in parallel to the feedback resistors. Due to the
THS4502 driving a capacitive load, small series resistors in
the output ensure stable operation. Further details of this and
other functions of the THS4502 may be found in its product
10pF(1)
+5V
+5V
392Ω
RS
392Ω
25Ω
25Ω
IN
IN
56.2Ω
VCM
THS4502
ADS5421
22pF
0.1µF
392Ω
CM
412Ω
10pF(1)
–5V
NOTES: Supply bypassing not shown. (1) Optional.
FIGURE 3. Using the THS4502 Differential Amplifier (Gain = 1) to Drive the ADS5421 in a DC-Coupled Configuration.
ADS5421
10
SBAS237D
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