Cirrus Logic AN330 REV2 User Manual
An330, Introduction
Copyright
Cirrus Logic, Inc. 2009
(All Rights Reserved)
Load Cell Measurement using the CS3001/02/11/12
Amplifiers with the CS5510/11/12/13 ADCs
Jerome E. Johnston
1. INTRODUCTION
Several circuits will be presented that use the CS3001/02/11/12 operational amplifiers with the CS5510/11/12/13
ADCs. The combination yields very high performance at relatively low power.
2. CS3001 AND CS5513 COMBINATION CIRCUIT
The first circuit illustrates the CS3001 single amplifier with the CS5513 A/D converter. The CS5513 converter in-
cludes an internal oscillator that sets the conversion rate at approximately 107 Sps.
In this circuit the load cell sensitivity is 1 mV / V. With 5 V excitation and full load, the output from the load cell will
be 5 mV.
The CS3001 amplifier is a chopper-stabilized amplifier with very low noise (6 nV /
√
Hz). The amplifier is unique in
that it has 300 dB of open loop gain. This permits the amplifier to be used in very-high-gain configurations and still
maintain excellent linearity.
Figure 1.
The circuit shows the CS3001 set with a gain of 408X. The gain is set by the ratio of the feedback resistor (71.5 k)
and the output resistance of the load cell (175 ohm which is set by the parallel combination of the two 350-ohm re-
sistors inside the bridge). Note that few op amps can support a gain of 408x (52 dB) and guarantee 16-bit or better
linearity; the CS3001 will be 20-bit linear due to its super-high open-loop gain. Note that the amplifier output is ref-
erenced to the common mode voltage of the load cell at about one half the supply, or 2.5 V. The 5 mV output of the
load cell is amplified by the op amp gain of 408 to yield an input to the ADC of 2.04 V, but this will be on top of the
2.5 volt common mode signal. The combination will result in a signal of about 4.504 volts on the AIN+ input of the
ADC.
The input span of the ADC is set by the VREF voltage. With 5 V into VREF, the span of the converter is set to ap-
proximately ±4 V, fully differential. The CS5513 is a 20-bit converter so its transfer function of 1,048,576 codes is
across 8 Vpp. Since the amplified load cell signal is only 2.04 volts the converter will output only about one quarter
+5 V
1 mV/V
-
+
350
Ω
200
Ω
200
Ω
0.047
μF
0.1
μF
VREF
AIN+
AIN1
V-
V+
CS
SDO
SCLK
CS5513
Microcontroller
+5 V
-
+
71.5 k
Ω
0.47
μF
0.1
μF
CS3001
100
Ω
0.1
μF
AN330
APR
‘
09
AN330REV2