17.8 A 190nW 33kHz RC oscillator with ±0.21% temperature stability and 4ppm long-term stability

Abstract

In wireless networks with a low duty cycle, the radio is operational for only a small percentage of the time. A sleep timer is used to synchronize the data transmission and reception. The total system power is then limited by the sleep power and the sleep timer frequency stability. Low-frequency crystal oscillators are a common choice for sleep timers due to their excellent long-term stability, frequency stability over temperature, and very low power consumption. However, the external crystal cost and board area are undesired. If an integrated oscillator is used as an alternative, the frequency variation must be minimized so the sleep time can be maximized.Several integrated oscillators with good frequency stability have been proposed in recent publications. In [1], feed-forward correction is used to achieve a stable frequency over temperature, but the voltage sensitivity and die area are large. In [2], self-chopping is used and excellent stability results are achieved at the cost of relatively high power consumption. In [3], offset cancellation is used to allow lower power operation while achieving stable temperature performance. However, long-term stability is limited by flicker noise to 20ppm for intervals over 0.5s. Also, low power systems may have supplies that are allowed to droop while in sleep, so improved frequency stability over supply variation is desired. In this work, an RC oscillator with improved long-term stability and frequency stability over supply voltage is presented.As shown in Fig. 17.8.1, the implemented RC oscillator consists of an RC network, and an inverting gain element from a resistor terminal to a capacitor terminal and another inverting gain element from the common resistor/capacitor terminal back to the resistor terminal. For high gain, the two inverting elements consist of three and five inverters, respectively. A simple regulator, consisting of an NMOS voltage follower and a replica inverter that is flipped and biased by a reference current, produces a local regulated supply for the inverters. This local supply is well below the standard core voltage for the technology. Therefore the inverters have very low average current because they are biased in weak inversion at their switching point.To analyze the frequency variation due to noise and other non-idealities, we define the relative inverter switching point as K SW =V SW /VDD LOCAL or, in other words, the ratio between the input=output inverter switching voltage V SW and the local supply voltage VDD LOCAL . Ignoring inverter delays and output resistance, the voltage at the common RC node will swing from (K SW +1)VDD LOCAL to (K SW −1)VDD LOCAL , as shown in Fig. 17.8.2. It can be shown that the ideal oscillation period equals When K SW =0.5, we get 50% duty cycle, oscillation period t 0 = RC•ln(9) ≈ 2.2RC and the lowest sensitivity to K SW variation.For a low frequency oscillator like this, the inverters driving the resistor and the capacitor will have much lower output resistance than the resistor R, and therefore contribute lit...

A UHF RFID chip solution with new oscillator calibration scheme and 16k bits EEPROM

“…7 shows the micro-photo of the tag chip, and the die size of the 16k bits EEPROM is 0.45 mm 2 , while the total tag chip area without test pad is 1 mm 2 . The energy storage capacitor Cs consumes a chip area of 0.28 mm 2 while the OSC including the calibration current array consumes a chip area of 0.0036 mm 2 , which is much smaller than the chip area OSC solution in [6] and [7]. The tag chip was packaged with a Chip-on-Board (COB) package and Flipchip package for function and performance test respectively.…”

“…However, these two solutions are featured by large frequency variation due to semiconductor process deviation. Without taking into the consideration of the process variation and the temperature coefficient of the resistor and capacitance, the OSC in [6] and [7] demonstrates a temperature frequency accuracy as small as AE0:21%. However, the frequency variation due to process drift is not provided.…”

“…Even the RC relaxation oscillators provided by [6] and [7] show good temperature stability, however, when taking into the consideration of the stringent chip area cost requirements of the tag chip, both these two solutions are discarded and a ring OSC with inverters act as the delay cells is adopted. The schematic of the ring OSC is shown in Fig.…”

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5.9 A 37μW dual-mode crystal oscillator for single-crystal radios

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