Datasheet

AD7715
Rev. D | Page 20 of 40
In unbuffered mode, the analog inputs look directly into the
input sampling capacitor, C
SAMP
. The dc input leakage current in
this unbuffered mode is 1 nA maximum. As a result, the analog
inputs see a dynamic load that is switched at the input sample
rate (see Figure 5). This sample rate depends on master clock
frequency and selected gain. C
SAMP
is charged to AIN(+) and
discharged to AIN(−) every input sample cycle. The effective
on-resistance of the switch, R
SW
, is typically 7 kΩ .
HIGH
IMPEDANCE
>1GΩ
R
SW
(7kΩ TYP)
C
SAMP
(10pF)
V
BIAS
SWITCHING FREQUENCY
DEPENDS ON
f
CLKIN
AND
SELECTED GAIN
AIN(+)
AIN(–)
08519-005
Figure 5. Unbuffered Analog Input Structure
C
SAMP
must be charged through R
SW
and through any external
source impedances every input sample cycle. Therefore, in
unbuffered mode, source impedances mean a longer charge
time for C
SAMP
, and this may result in gain errors on the part.
Table 24 shows the allowable external resistance/capacitance
values, for unbuffered mode, such that no gain error to the
16-bit level is introduced on the part. Note that these capacitances
are total capacitances on the analog input, external capacitance
plus 10 pF capacitance from the pins and lead frame of the device.
Table 24. External R, C Combination for No 16-Bit Gain
Error (Unbuffered Mode Only)
External Capacitance (pF)
Gain
10 50 100 500 1000 5000
1 152 kΩ 53.9 kΩ 31.4 kΩ 8.4 kΩ 4.76 kΩ 1.36 kΩ
2 75.1 kΩ 26.6 kΩ 15.4 kΩ 4.14 kΩ 2.36 kΩ 670 Ω
32 16.7 kΩ 5.95 kΩ 3.46 kΩ 924 Ω 526 Ω 150 Ω
128 16.7 kΩ 5.95 kΩ 3.46 kΩ 924 Ω 526 Ω 150 Ω
In buffered mode, the analog inputs look into the high impedance
inputs stage of the on-chip buffer amplifier. C
SAMP
is charged via
this buffer amplifier such that source impedances do not affect
the charging of C
SAMP
. This buffer amplifier has an offset leakage
current of 1 nA. In this buffered mode, large source impedances
result in a small dc offset voltage developed across the source
impedance but not in a gain error.
Input Sample Rate
The modulator sample frequency for the AD7715 remains at
f
CLK IN
/128 (19.2 kHz @ f
CLK IN
= 2.4576 MHz) regardless of the
selected gain. However, gains greater than 1 are achieved by a
combination of multiple input samples per modulator cycle and
a scaling of the ratio of reference capacitor to input capacitor. As
a result of the multiple sampling, the input sample rate of the device
varies with the selected gain (see Table 25). In buffered mode, the
input is buffered before the input sampling capacitor. In unbuffered
mode, where the analog input looks directly into the sampling
capacitor, the effective input impedance is 1/C
SAMP
× f
S
where
C
SAMP
is the input sampling capacitance and f
S
is the input
sample rate.
Table 25. Input Sampling Frequency vs. Gain
Gain Input Sampling Frequency (f
S
)
1 f
CLK IN
/64 (38.4 kHz @ f
CLK IN
= 2.4576 MHz)
2 2 × f
CLK IN
/64 (76.8 kHz @ f
CLK IN
= 2.4576 MHz)
32 8 × f
CLK IN
/64 (307.2 kHz @ f
CLK IN
= 2.4576 MHz)
128 8 × f
CLK IN
/64 (307.2 kHz @ f
CLK IN
= 2.4576 MHz)
Bipolar/Unipolar Inputs
The analog input on the AD7715 can accept either unipolar or
bipolar input voltage ranges. Bipolar input ranges do not imply
that the part can handle negative voltages on its analog input
since the analog input cannot go more negative than −30 mV to
ensure correct operation of the part. The input channel is fully
differential. As a result, the voltage to which the unipolar and
bipolar signals on the AIN(+) input are referenced is the voltage
on the respective AIN(−) input. For example, if AIN(−) is
2.5 V and the AD7715 is configured for unipolar operation with
a gain of 2 and a V
REF
of 2.5 V, the input voltage range on the
AIN(+) input is 2.5 V to 3.75 V. If AIN(−) is 2.5 V and the
AD7715 is configured for bipolar mode with a gain of 2 and a
V
REF
of 2.5 V, the analog input range on the AIN(+) input is
1.25 V to 3.75 V (that is, 2.5 V ± 1.25 V). If AIN(−) is at AGND,
the part cannot be configured for bipolar ranges in excess of
±30 mV.
Bipolar or unipolar options are chosen by programming the
B
/U bit of the setup register. This programs the channel for
either unipolar or bipolar operation. Programming the channel
for either unipolar or bipolar operation does not change any
of the input signal conditioning; it simply changes the data
output coding and the points on the transfer function where
calibrations occur.