Datasheet

+
–
R
SOURCE
R
SOURCE
R
S
R
S
C
BYPASS
AVDD
GND
ADS7945
AINxP
AINxN
+
+
Signal Source
GND
ADS7945
ADS7946
SBAS539B –JUNE 2011– REVISED SEPTEMBER 2011 www.ti.com
DRIVING AN ADC WITHOUT A DRIVING OP AMP
For some low input signal bandwidth applications, such as battery power monitoring or mains monitoring, it is not
required to operate an ADC at high sampling rates. In fact, it is desirable to avoid using a driving op amp from a
cost perspective. In these cases, the ADC input sees the impedance of the signal source (such as a battery or
mains transformer). This section elaborates the effects of source impedance on sampling frequency.
Equation 1 can be rewritten as Equation 4:
Sampling Time = Filter Time Constant × Settling Resolution × ln(2) (4)
As shown in Figure 87, it is recommended to use a bypass capacitor across the positive and negative ADC input
terminals.
Figure 87. Driving an ADS7945 ADC Without a Driving Op Amp
Source impedance (2 × R
SOURCE
+ 2 × R
S
) with (C
BYPASS
+ C
SAMPLE
) acts as a low-pass filter with Equation 5:
Filter Time Constant = 2 × (R
SOURCE
+ R
S
) × (C
BYPASS
+ C
SAMPLE
)
where:
C
SAMPLE
is the internal sampling capacitance of the ADC (equal to 32 pF). (5)
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