Datasheet

AVDD
(1)f
MOD
=76.8kHz
Z =500MW
EFF
(1)
REFP REFN
AVDD
ESD
Protection
C
BUF
CLK_DETECT
Internal
Oscillator
MUX
ToADC
S
S0 S1
EN
CLKIN
Z
EFF
+
1
(2)(76.8kHz)(13fF)
+ 500MW
Z
EFF
+
1
2f
MOD
C
BUF
ADS1230
www.ti.com SBAS366B –OCTOBER 2006–REVISED SEPTEMBER 2012
VOLTAGE REFERENCE INPUTS ESD diodes protect the reference inputs. To prevent
(REFP, REFN) these diodes from turning on, make sure the voltages
on the reference pins do not go below GND by more
The voltage reference used by the modulator is
than 100mV, and likewise, do not exceed AVDD by
generated from the voltage difference between REFP
100mV:
and REFN: V
REF
= REFP – REFN. The reference
inputs use a structure similar to that of the analog GND – 100mV < (REFP or REFN) < AVDD + 100mV
inputs. In order to increase the reference input
impedance, a switching buffer circuitry is used to
CLOCK SOURCES
reduce the input equivalent capacitance. The
The ADS1230 can use an external clock source or
reference drift and noise impact ADC performance. In
internal oscillator to accommodate a wide variety of
order to achieve best results, pay close attention to
applications. Figure 20 shows the equivalent circuitry
the reference noise and drift specifications. A
of the clock source. The CLK_DETECT block
simplified diagram of the circuitry on the reference
determines whether the crystal oscillator/external
inputs is shown in Figure 19. The switches and
clock signal is applied to the CLKIN pin so that the
capacitors can be modeled approximately using an
internal oscillator is bypassed or activated. When the
effective impedance of:
CLKIN pin frequency is above ~200kHz, the
CLK_DETECT output goes low and shuts down the
internal oscillator. When the CLKIN pin frequency is
below ~200kHz, the CLK_DETECT output goes high
Where:
and activates the internal oscillator. It is highly
f
MOD
= modulator sampling frequency (76.8kHz)
recommended to hard-wire the CLKIN pin to ground
C
BUF
= input capacitance of the buffer
when the internal oscillator is chosen.
For the ADS1230:
Figure 20. Equivalent Circuitry of the Clock
Source
An external clock may be used by driving the CLKIN
pin directly. The Electrical Characteristics table shows
the allowable frequency range. The clock input may
be driven with 5V logic, regardless of the DVDD or
AVDD voltage.
Figure 19. Simplified Reference Input Circuitry
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