Application Note. Lock-in Milliohmmeter

Transcription

1 Applicatin Nte AN2207 Lck-in Millihmmeter Authr: Oleksandr Karpin Assciated Prject: Yes Assciated Part Family: CY8C24xxxA, CY8C27xxx PSC Designer Versin: 4.1 SP1 Assciated Applicatin Ntes: AN2028, AN2044, AN2120, AN2148, AN2158 Abstract This Applicatin Nte describes a lck-in millihmmeter. A millihmmeter is a handy tl fr tasks such as measuring a switch s cntact resistance and checking the trace resistance n a PCB. Lck-in mdulatin technlgy, which is used in this design, reduces nise and allws users t measure resistance with a sufficiently lw test current. Intrductin A millihmmeter is used fr measuring very small resistances. Examples include trace resistance n a PCB, shrted traces, and cntact resistance f a switch r cnnectr. The main issues fr the millihmmeter design are: Hw t get stable resistance readings with a suitably lw test current? Fr example, 50mA1m 50V. This means, that the millihmmeter wuld have t be able t measure and display vltages in the range f fifty micrvlts per cunt in a stable and repeatable manner. Active semicnductr devices exhibit a lw frequency phenmenn knw as flicker nise, ften called 1/f nise. Hw is flicker nise eliminated in the measurement? Lck-in mdulatin (als called synchrnus mdulatin/demdulatin) prvides high gain fr the signal at a specific frequency and phase while reducing ther signals. Other signals include 1/f nise. Therefre, the lck-in mdulatin technique is a gd answer t issues assciated with millihmmeter design. The millihmmeter technical specificatins are given in Table 1. Table 1. Millihmmeter Specificatins Item Pwer Supply Vltage Pwer Cnsumptin Mdulatin Frequency Measured Resistance Range Specificatin 5V 55 ma 5 khz Measurement Errr 0,5% Measurement Vltage at Prbes 0,2V Display Type 0 10 hm Cmpatible with industry standard HD Based LCD Mdule Measure Resistance The technique used t measure resistance is explained in detail in Applicatin Nte AN2028 Standard - Ohmmeter. A millihmmeter must be able t measure resistance with reslutin f several Millihms. Therefre, the cnnectin frm the device t the measured resistr is calculated and intrduced as an errr in the result. 8/4/2005 Revisin A - 1 -

2 T cancel this errr, a 4-wire cnnectin is used. In the system shwn in Figure 1, the drp in wiring resistances Rc and Rd will be negligible due t the high input impedance f the measurement system. Equatin (1) shws hw the measured resistance is determined: V 2 V3 Rt Rref (1) V0 V1 As seen in the equatin, the vltage drp acrss Ra (V 1 -V 2 ) and Rb (V 3 -V 4 ) cancel ut and is therefre nt affected by the length and resistance f the cnnecting cable. A vltage drp acrss the resistr device uses an instrumental amplifier. Therefre, the measured resistance is determined using Equatin (2): V Rt R t Rref (2) VRref V Rt is vltage drp acrss the measured resistr Rt. is vltage drp acrss the V Rref R ref reference resistr. When we have t measure small vltages, the prblems f ffset, gain and signal-t-nise rati cme int cnsideratin. T vercme these prblems we will use: Crrelated duble sampling (CDS). Full-scale calibratin. Lck-in mdulatin technique. Analg and digital filters. Crrelated Duble Sampling (CDS): Crrelated duble sampling eliminates ffset errrs present in the signal cnditining circuit and the ADC. The fllwing steps are invlved. 1. Shrt the input. Measure ADC utput. Stre as zer. 2. Cnnect inputs t measured resistr. Measure ADC utput. Stre as signal. 3. Subtract zer frm signal. This methd eliminates gain errrs present in the signal cnditining circuit and the ADC. With CDS and reference calibratin, the measured resistance can be calculated by the fllwing equatin: V Rt Vzer R t Rref (3) VRref Vzer As is shwn in Equatin (3), any ffset errrs in the measurement system are remved by the subtractin f tw measured vltages. This is CDS. The rati f these tw different values remves any measurement path gain errr. Reference Full-Scale Calibratin: The parameter f designates reference R ref resistance fr this device. Fr better reslutin resistance, millihms are used as the unit f measure. This leaves the measurement errr t be determined by nly the accuracy f. R ref Lck-in Mdulatin Technlgy: This technique prvides a signal gain at a specific frequency and phase, while reducing ther signals, including nise. It invlves: Selecting the specific signal. Mdulating the specific signal. Amplifying the specific signal. Filtering the specific signal with a band pass filter (BPF). Demdulating the specific signal. The demdulated signal btains a signal-t-nise rati that is greater than ne (SNR>1). This methd eliminates all f the drift prduced inside the lck-in circuit. Fr a backgrund n switched capacitr mdulatrs, review Applicatin Nte AN2044 Standard - Signal Rectificatin, using Switched Capacitr Mdulatrs. Analg and Digital Filters: Analg filters limit the bandwidth and reduce the effect f nise n the input signal. A digital filter rutine simply cllects several ADC samples then finds the average. This is used t determine the average measurement f the ADC nise. Millihmmeter Blck Diagram Figure 1 depicts the millihmmeter flwchart. The carrier generatr prduces the mdulatin signal that is applied t a sense resistr. High impedance analg multiplexers allw measurement f the sense resistr and shrting f the inputs t measure systems errr. The clumn multiplexer allws users t autmatically shrt the input (in prgram) fr CDS executin. 8/4/2005 Revisin A - 2 -

3 Carrier Generatr V Rref Analg Multiplexer U1 U2 Ra Rc Clumn Multiplexer Instrumentatinal Amplifier Band Pass Filter Synchrnus Demdulatr Lw Pass Filter Integrating ADC Rt U3 Rb Rd Analg Multiplexer Digital Filter Sftware Implemented Millihm Calculatin Millihm Test Text LCD RS232 Figure 1. Millihmmeter Flwchart A vltage drp taken acrss the resistr device s instrumental amplifier is used. It amplifies the signal and makes it relative t AGND. The instrumental amplifier utput is fed t a band pass filter (BPF). After passing thrugh a band pass filter, the utput is amplified and sent thrugh a filtered signal. Then a synchrnus demdulatr rectifies it. The demdulatr reference signal is set t the mdulatin signal. The demdulatr utput is then fed t a lw pass filter (LPF). The lw pass filter amplifies and filters this rectified signal t prduce a filtered signal. This filtered signal is sampled directly by the integrating ADC. The integrating ADC utilized has a lw pass bandwidth set by the utput sample rate. The Nyquist sampling rate is at the input sample clck rate. Fr an n-bit cnverter, this is 2 n times higher than the utput rate and effectively eliminates any biasing prblems. Using the PSC TM device, we can make a demdulatr based n a lw pass filter, which prvides mre accurate utput data. Nte that the ADC cnversin time is exactly the same as the carrier-signal perid s integer number s that the ADC s gain frequency matches the carrier signal harmnics. The fllwing prcessing f ADC data stream is implemented in the firmware. The sftware calculates the measured resistance in millihms as shwn in Equatin (3). The digital filter rutine cllects several measured resistances and then finds the average. The result f resistance average utput is then sent t the LCD. Fr debugging purpses and external resistance evaluatin, intermediate data can be sent via RS232 prt. Device Schematic The cmplete millihmmeter schematic is shwn in Figure 2 fr measurement, and Figure 3 fr display and utput. R 1 is the reference resistr. As shwn in figures 2 and 3, prbes frm J 1 cnnect with measured resistr Rt. The mdulatin frequency signal frm prt P2[0] ges thrugh series-drpping resistr R 2, R 1 and the measured resistr. The R 2 is destined t reduce current and vltage f measurements. Vltages frm the measured and reference resistr R 1 take the readings t the prts P0[0], P0[1], P0[6], P0[7] with high input impedance. They cme thrugh high pass filters based n C 8 R 4, C 7 R 5, C 5 R 6, and C 2 R 7 with reference t AGND. The R 10 R 11 C 14 determines the analg grund ptential. T prvide a lw vltage at prbes, dide D4 is used. Dides D4 and D5 prtect the device frm the utside high vltage. Speaker U4 makes a vice signal t indicate that the device measurement is in range. The millihmmeter used is cmpatible with the industry standard HD44780-Based LCD Mdule, which is a 16-character by 2-line display driver, U3. The ptinal RS232 Level Translatr U2 with aut-shutdwn can be used fr sending the ADC raw data r measured resistance values t the external PC. The test pints TP1 and TP2 are intended fr bserving sme PSC device internal signals. TP1 is utput f the band pass filter. TP2 is utput f the lw pass filter. The millihmmeter can be pwered frm nnstabilized 6-12V DC/AC supply with maximum current f 100 ma. Nrmal peratinal current is significantly less. 8/4/2005 Revisin A - 3 -

4 R ref Figure 2. Millihmmeter Schematic: CPU and Measuring Equipment Figure 3. Millihmmeter Schematic: LCD, RS232 and Analg Cmpnents 8/4/2005 Revisin A - 4 -

5 PSC Internals The PSC internal structure is shwn in Figure 4. The instrumental amplifier is placed in ACB00 and ACB01. The amplifier is cnnected with BPF, which is placed in ASC10 and ASD20. The BPF utput is sent via Prt = P0[3] t checkpint TP1 fr testing and debugging purpses. The filter utput signal is passed t a synchrnus rectifier, which is build arund LPF and is placed in ASD11 and ASC21. The rectifier reference cmes frm GlbalOut_Even0. The LPF utput is sent via Prt = P0[5] t checkpint TP2 fr testing and debugging purpses. Nte that the clumn buffers BUF0 and BUF1 can be disabled t save pwer in the prductin release. In the 5Vpwered millihmmeter, the AGND level is set t Vdd/2 BandGap. The LPF utput is cnnected with ADC input. In this design, the incremental ADC is used. The ADC reslutin is set t 14 bits, but the integratin time is adjusted t precisely equal the integer number f the mdulatin signal in the firmware by directly writing t the ADC 17-bit PWM Perid and Cmpare registers. As a result, the effective reslutin is 13.6 bits. The mdulatin frequency is set t 5 khz, filter versample rati is 50, ADC integratin time is 50 ms, r 250 mdulatin signal perids, and the ADC sample frequency is very clse t 20 Hz. All switched capacitr mdules use the same clumn frequency t prevent errrs. Figure 4. PSC Internal User Mdule Cnfiguratin 8/4/2005 Revisin A - 5 -

6 The serial transmitter (placed in DCB13), alng with the baud-rate timer (placed in DCB12), is used fr transmitting debug ADC sample stream r ther infrmatin t the PC. Bth can be mitted in the prductin release. The timer (placed in DBB00) frms the mdulatin signal; the timer clck surce is VC3. The LCD interface is implemented in sftware. The alarm cunter (placed in DCB02) is used fr signal-t-speaker generatin. Nte that the prject can be used with the lwer-cst CY8C24xxxA PSC device withut any prblems. The serial transmitter and alarm cunter mdules shuld be eliminated, because the debug functinality is nt required in the prductin release. A synchrnus rectifier shuld be built arund the cnfigurable switched capacitr blck. The prject must use an incremental ADC with a reslutin f 13 bits. This technique is explained in detail in Applicatin Nte AN Standard - Optical PulsOmeter with PSC. The Sftware The millihmmeter sftware cnsists mainly f three cmpnents: readadc prcessadc main lp The lgic f the prgram is explained by prgram flwcharts in figures 5, 6 and 7. Start getadcsamples(1) Is Data Ready? N Yes Return ADCData Figure 5. Prgram Flwchart fr readadc 8/4/2005 Revisin A - 6 -

7 Start AMuxPs_In(PORT0_0) AMuxNeg_In(PORT0_1) Vref=readAdc Flip cl1_mux n cl0 Vzer=readAdc Vref=Vref-Vzer Flip cl1_mux n cl1 AMuxPs_In(PORT0_6) AMuxNeg_In(PORT0_7) Vt=readAdc Flip cl1_mux n cl0 Vzer=readAdc Vt=Vt-Vzer Flip cl1_mux n cl1 lrt=vt*1000/vref Rt=(int)lRt Return Rt Figure 6. Prgram Flwchart fr prcessadc 8/4/2005 Revisin A - 7 -

8 Start cunt=0; ps=0; Rt_Sample = 10; MAX_Rref=955 iart[ps]=prcessadc Yes ps==rt_sample-1 N ps=0 ps++ cunt<rt_sample N Yes cunt++ lrt=0 i=0 lrt=lrt+iart[i] i++ Yes i<rt_sample-1 N lrt=lrt/cunt Yes lrt<=max_rref N LCD_ut(lRt) LCD_ut( Errr ) Figure 7. Prgram Flwchart fr main lp 8/4/2005 Revisin A - 8 -

9 Applicatin Dmains Using tw additinal buttns, we can set the resistance range f dynamic speaker actuatin. After this, we can easily check the trace resistance n a PCB. The millihmmeter can be used fr measuring shrted traces as shwn in Figure Figure 8. Tracking Shrted Traces Device Phtgraph Figure 9. Bard Phtgraph 8/4/2005 Revisin A - 9 -

10 Abut the Authr Name: Title: Backgrund: Cntact: Oleksandr Karpin Pst-Graduate Student Oleksandr earned his cmputer-engineering diplma in 2001 frm Natinal University "Lvivska Plytechnika" (Lviv, Ukraine), then studied as a pst-graduate student at the same university. His interests include embedded systems design and new technlgies. karpin@plynet.lviv.ua Cypress Semicnductr nd Street SW, Building D Lynnwd, WA Phne: Fax: Cpyright 2005 Cypress Semicnductr Crpratin. All rights reserved. "Prgrammable System n Chip," PSC, PSC Designer and PSC Express are trademarks f Cypress Semicnductr Crp. All ther trademarks r registered trademarks referenced herein are the prperty f their respective wners. The infrmatin cntained herein is subject t change withut ntice. Made in the U.S.A. 8/4/2005 Revisin A