Datasheet

t
SAMPLE
ON
OFF
S
1
S
2
OFF
ON
S
1
S
1
AVSS + 1.3V
R
AIN
= R
effB
|| 2R
effA
AVSS + 1.3V
R
effA
= 190k
Ω
R
effB
= 78k
Ω
(f
CLK
= 16MHz)
R
effA
= 190k
Ω
ADCINN
ADCINP
C
A1
= 0.65pF
C
B
= 1.6pF
C
A2
= 0.65pF
ADCINN
S
2
AVSS + 1.3V
S
2
AVSS + 1.3V
ADCINP
Equivalent
Circuit
R
eff
= t
SAMPLE
/C
X
NOTE: ESD input diodes not shown.
ADS1258-EP
www.ti.com SBAS445D –MARCH 2009– REVISED MARCH 2011
ADC INPUTS As with the multiplexer and reference inputs, ESD
diodes protect the ADC inputs. To keep these diodes
The ADS1258 ADC inputs (ADCINP, ADCINN)
from turning on, make sure the voltages on the input
measure the input signal using internal capacitors
pins do not go below AVSS by more than 100 mV,
that are continuously charged and discharged. The
and likewise do not exceed AVDD by more than 100
left side of Figure 38 shows a simplified schematic of
mV.
the ADC input circuitry; the right side of Figure 38
shows the input circuitry with the capacitors and
switches replaced by an equivalent circuit. Figure 37
shows the ON/OFF timings of the switches shown in
Figure 38. S
1
switches close during the input
sampling phase. With S
1
closed, C
A1
charges to
ADCINP, C
A2
charges to ADCINN, and C
B
charges to
(ADCINP – ADCINN). For the discharge phase, S
1
opens first and then S
2
closes. C
A1
and C
A2
discharge
to approximately AVSS + 1.3 V and C
B
discharges to
0V. This two-phase sample/discharge cycle repeats
with a period of t
SAMPLE
= 2/f
CLK
.
Figure 37. S
1
and S
2
Switch Timing for Figure 38
The charging of the input capacitors draws a transient
current from the source driving the ADS1258 ADC
inputs. The average value of this current can be used
to calculate an effective impedance (R
eff
) where R
eff
=
V
IN
/I
AVERAGE
. These impedances scale inversely with
f
CLK
. For example, if f
CLK
is reduced by a factor of
two, the impedances double.
Figure 38. Simplified ADC Input Structure
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