AD7714
ANALOG INPUT
Analog Input Ranges
CSAMP must be charged through RSW and through any external
source impedances every input sample cycle. Therefore, in unbuf-
fered mode, source impedances mean a longer charge time for
CSAMP and this may result in gain errors on the part. Table XII
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. Table XIII shows the allowable external
resistance/capacitance values, once again for unbuffered mode,
such that no gain error to the 20-bit level is introduced.
The AD7714 contains six analog input pins (labelled AIN1 to
AIN6) which can be configured as either three fully differential
input channels or five pseudo-differential input channels. Bits
CH0, CH1 and CH2 of the Communications Register configure
the analog input arrangement and the channel selection is as
outlined previously in Table VII. The input pairs (either differ-
ential or pseudo-differential) provide programmable-gain, input
channels which can handle either unipolar or bipolar input
signals. It should be noted that the bipolar input signals are
referenced to the respective AIN(–) input of the input pair.
Table XII. External R, C Combination for No 16-Bit Gain
Error (Unbuffered Mode Only)
In unbuffered mode, the common-mode range of these inputs is
from AGND to AVDD provided that the absolute value of the analog
input voltage lies between AGND – 30 mV and AVDD + 30 mV.
This means that in unbuffered mode the part can handle both
unipolar and bipolar input ranges for all gains. In buffered
mode, the analog inputs can handle much larger source imped-
ances, but the absolute input voltage range is restricted to be-
tween AGND + 50 mV to AVDD – 1.5 V which also places
restrictions on the common-mode range. This means that in
buffered mode there are some restrictions on the allowable gains
for bipolar input ranges. Care must be taken in setting up the
common-mode voltage and input voltage range so that the
above limits are not exceeded, otherwise there will be a degrada-
tion in linearity performance.
Gain
External Capacitance (pF)
50 100 500 1000
368 kΩ 90.6 kΩ 54.2 kΩ 14.6 kΩ 8.2 kΩ 2.2 kΩ
0
5000
1
2
4
177.2 kΩ 44.2 kΩ 26.4 kΩ 7.2 kΩ
82.8 kΩ 21.2 kΩ 12.6 kΩ 3.4 kΩ
4 kΩ
1.94 kΩ 540 Ω
1.58 kΩ 880 Ω 240 Ω
1.12 kΩ
8–128 35.2 kΩ 9.6 kΩ 5.8 kΩ
Table XIII. External R, C Combination for No 20-Bit Gain
Error (Unbuffered Mode Only)
Gain
External Capacitance (pF)
0
50
100
500
1000
5000
In unbuffered mode, the analog inputs look directly into the
7 pF input sampling capacitor, CSAMP. The dc input leakage
current in this unbuffered mode is 1 nA maximum. As a result,
the analog inputs see a dynamic load which is switched at the
input sample rate (see Figure 3). This sample rate depends on
master clock frequency and selected gain. CSAMP is charged to
AIN(+) and discharged to AIN(–) every input sample cycle.
The effective on-resistance of the switch, RSW, is typically 7 kΩ.
1
2
4
290 kΩ
141 kΩ
63.6 kΩ 16 kΩ
69 kΩ
33.8 kΩ 20 kΩ
40.8 kΩ 10.4 kΩ 5.6 kΩ 1.4 kΩ
5 kΩ
2.4 kΩ
1.1 kΩ
2.8 kΩ 700 Ω
1.34 kΩ 340 Ω
9.6 kΩ
4.4 kΩ
8–128 26.8 kΩ 7.2 kΩ
600 Ω
160 Ω
In buffered mode, the analog inputs look into the high impedance
inputs stage of the on-chip buffer amplifier. CSAMP is charged via
this buffer amplifier such that source impedances do not affect
the charging of CSAMP. This buffer amplifier has an offset leak-
age current of 1 nA. In this buffered mode, large source imped-
ances result in a dc offset voltage developed across the source
impedance but not in a gain error.
AIN(+)
R
(7k⍀ TYP)
SW
HIGH
Input Sample Rate
IMPEDANCE
>1G⍀
The modulator sample frequency for the AD7714 remains at
fCLK IN/128 (19.2 kHz @ fCLK 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 capaci-
tor. As a result of the multiple sampling, the input sample rate
of the device varies with the selected gain (see Table XIV). 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 imped-
ance is 1/CSAMP × fS where CSAMP is the input sampling capaci-
tance and fS is the input sample rate.
C
SAMP
AIN(–)
(7pF )
V
BIAS
SWITCHING FREQUENCY DEPENDS ON
fCLKIN AND SELECTED GAIN
Figure 3. Unbuffered Analog Input Structure
–20–
REV. C
ADI [ ADI ]
