ADS1230
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SBAS366–OCTOBER 2006
OVERVIEW
64) or 0mV to +19.5mV (Gain = 128). The inputs of
the ADS1230 are protected with internal diodes
connected to the power-supply rails. These diodes
clamp the applied signal to prevent it from damaging
the input circuitry.
The ADS1230 is a precision, 20-bit ADC that
includes
a
low-noise PGA, internal oscillator,
third-order delta-sigma (∆Σ) modulator, and
fourth-order digital filter. The ADS1230 provides a
complete front-end solution for bridge sensor
applications such as weigh scales, strain guages,
and pressure sensors.
CAP
450W
RINT
AINP
18pF
Clocking can be supplied by an external clock or by
a precision internal oscillator. Data can be output at
10SPS for excellent 50Hz and 60Hz rejection, or at
80SPS when higher speeds are needed. The
ADS1230 is easy to configure, and all digital control
is accomplished through dedicated pins; there are no
A1
R
F1
Gain = 1 or 2
R1
A3
ADC
RF2
registers to program.
A simple two-wire serial
interface retrieves the data.
RINT
A2
450W
ANALOG INPUTS (AINP, AINN)
AINN
18pF
The input signal to be measured is applied to the
input pins AINP and AINN. The ADS1230 accepts
differential input signals, but can also measure
unipolar signals. When measuring unipolar (or
single-ended signals) with respect to ground,
connect the negative input (AINN) to ground and
connect the input signal to the positive input (AINP).
Note that when the ADS1230 is configured this way,
only half of the converter full-scale range is used,
since only positive digital output codes are produced.
CAP
Figure 18. Simplified Diagram of the PGA
Bypass Capacitor
By applying a 0.1µF external capacitor (CEXT) across
two capacitor pins combined with the internal 2kΩ
resistor RINT (on-chip), a low-pass filter with a corner
frequency of 720Hz is created to bandlimit the signal
path before the modulator input. This low-pass filter
serves two purposes. First, the input signal is
bandlimited to prevent aliasing as well as to filter out
the high-frequency noise. Second, it attenuates the
chopping residue from the amplifier to improve
temperature drift performance. It is not required to
use high-quality capacitors (such as ceramic or
LOW-NOISE PGA
The ADS1230 features a low-drift, low-noise PGA
that provides a complete front-end solution for bridge
sensors. A simplified diagram of the PGA is shown in
Figure 18. It consists of two chopper-stabilized
amplifiers (A1 and A2) and three accurately-matched
resistors (R1, RF1, and RF2), which construct a
differential front-end stage with a gain of 64, followed
by gain stage A3 (Gain = 1 or 2). The PGA inputs
are equipped with an EMI filter, as shown in
Figure 18. The cutoff frequency of the EMI filter is
19.6MHz. By using AVDD as the reference input, the
bipolar input ranges from –39mV to +39mV (Gain =
64) or –19.5mV to +19.5mV (Gain = 128), and the
unipolar input ranges from 0mV to +39mV (Gain =
tantalum capacitors) for
a general application.
However, high-quality capacitors such as poly are
recommended for high-linearity applications.
10
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